I believe that there are two possible answers to this question. One answer focuses on the more immediate question of what causes or triggers each imaginary fantasy, and the other answer looks toward the deeper seated root cause of why Mitty is having so many daydreams in the first place.
Mitty's daydreams aren't random. They are triggered by something in the environment around him. When he sees a hospital, he imagines that he is an expert surgeon. When he hears about a court trial, he is a fantastic lawyer. When he sees pictures of bombers, Mitty becomes a brave pilot. When he lights up a cigarette, he imagines himself standing before a firing squad.
The fact that Mitty is having daydreams shouldn't surprise or shock readers. We've all had daydreams, and more than likely, they involve us doing something in and around the environment that we were present in at that moment. What readers should notice is just how often Mitty has his daydreams. They are constant. It's possible that Mitty has some kind of underlying mental disorder and simply can't prevent his mind from wandering all over the place. It might be interesting to explore what mental disorder is the root cause of Mitty's daydreams.
If his daydreams are not caused by a medical condition, then most readers come from the angle that the underlying reason for his daydreams is that Mitty is seeking escapism. He is treated as somewhat inept in his real life by his wife. She orders him around about everything, and his daydreams clearly put himself in a position of power in which he is the alpha giving everyone else the orders. Mitty would simply rather live in his dreams than in his reality, and that is what triggers daydream after daydream.
Further Reading
Walter Mitty's daydreams are a form of escape from the boredom of everyday life. His real life is characterized by boredom, routine, lack of satisfaction, and a nagging wife. His dreams, on the other hand, involve being a bomber pilot, facing a firing squad, and being on the witness stand in the courtroom revealing that he is a talented marksman, and having an attractive young woman fall into his arms.
Real life is juxtaposed against a world of excitement and fantasy, and since Walter is unable to create any excitement in his real life, he escapes into these fantasy worlds at every given opportunity.
His wife, who seems to have no understanding of her husband's lack of satisfaction with his life, treats his fantasies with disdain, informing him at one point that she will need to take his temperature when they get home. I would argue that his unhappiness in his marriage makes his boredom with his life worse, which in turn leads to more of the daydreams which his wife so derides.
In a nutshell, Walter's daydreams are caused by the same factor which makes some people escape into books, others gamble, and yet others have extramarital affairs. He is looking for...
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an escape from the mundane nature of his everyday life.
Further Reading
All of Walter's daydreams are ultimately caused by his dull daily existence, but small moments in the story also show the small moments that trigger his imagination.
For example, Walter drives past the hospital, which makes him imagine being a famous surgeon called in for an important case. He hears a paper boy talking about a famous court case, and then he imagines himself being called to the stand in a murder investigation. While it's not clear if all of his daydreams are caused by things like this (his firing squad daydream, for example, seems to have no direct connection to his surroundings), it is clear that his overactive imagination is stimulated by what he sees and hears, even if he himself is not aware of the fact.
In the short story "The Secret Life of Walter Mitty," it's clear that Mitty's demanding wife and overall disappointing life are the causes of his daydreams.
Each daydream is caused by something his wife did or said.
For example, the first daydream of him piloting a submarine is interrupted by his wife who tells him to slow down and then, in a nagging voice, says, "What are you driving so fast for?" While this event happens after his daydream, it's still his wife who precipitates it.
Then she suggests that he get medical attention for his daydreams.
Each daydream deals with his attempt to escape his wife's control, but at the end, as he has probably done his whole marriage, he succumbs to both his wife and his imaginary firing squad.
What effect do Walter Mitty's daydreams have on the story "The Secret Life of Walter Mitty?"
In James Thurber's entertaining short story "The Secret Life of Walter Mitty," the main character's daydreams remove him from the irritations of his mundane life by providing him with an heroic alternate reality. For example, at the beginning of the story, he imagines himself the pilot of a Navy plane that is about to enter a storm:
"The crew, bending to their various tasks in the huge, hurtling eight-engined Navy hydroplane, looked at each other and grinned. 'The Old Man’ll get us through,' they said to one another. 'The Old Man ain’t afraid of Hell!'". . .
Walter Mitty is actually driving with his wife, who ends his reverie by telling him that he is driving too fast. After his wife reminds him to put on his gloves, he imagines that he is a surgeon:
"A door opened down a long, cool corridor and Dr. Renshaw came out. He looked distraught and haggard. 'Hello, Mitty,' he said. 'We’re having the devil’s own time with McMillan, the millionaire banker and close personal friend of Roosevelt. Obstreosis of the ductal tract. Tertiary. Wish you’d take a look at him.' 'Glad to,' said Mitty."
In this incident, Mitty removes himself from the drab reality of his wife's reminder that he should put on his gloves by imagining that his gloves make him a respected doctor. His daydreams help him make up for the boring reality of his everyday life and for the way in which his relationship with his wife makes him feel less-than-powerful. His daydreams make the story more entertaining and lively, and they also allow the reader to understand a great deal about the way in which Mitty experiences his life.
What causes Walter Mitty's fantasies?
It is interesting to identify just the things that evoke Walter Mitty's fantasies throughout the story. One which can be easily overlooked is especially subtle and seems intended to tie the story together with a beginning and an ending. In the opening paragraph Commander Mitty, in full-dress uniform, is not driving an automobile to Waterbury on a shopping trip but piloting a "huge, hurtling eight-engined Navy hydroplane" through stormy seas, shouting such absurdities as "Switch on No. 8 auxiliary!" and "Full strength in No. 3 turret!" An officer warns him, "We can't make it, sir. It's spoiling for a hurricane, if you ask me." "I'm not asking you, Lieutenant Berg," said the Commander. "Throw on the power lights! Rev her up to 8,500! We're going through!" No such boat ever existed, of course, and a hydroplane with eight engines sounds like an utter impossibility. But what is most interesting is the warning that "It's spoiling for a hurricane."
Walter Mitty must have noticed a few raindrops on his windshield, giving warning that it was going to rain before they got their errands run and returned home. At the very end of the story, while Mitty is waiting for his wife outside the drugstore, "Walter Mitty lighted a cigarette. It began to rain, rain with sleet in it. He stood up against the wall of the drugstore, smoking." This is an admirably artistic way to begin and end the story. The approaching rainstorm and the rain and sleet finally falling seem to round out what could otherwise be an open-ended tale with many other untold fantasies preceding and following those that are dramatized. The end is vaguely reminiscent of James Joyce's story "The Dead," which ends with this beautiful passage:
A few light taps upon the pane made him turn to the window. It had begun to snow again. He watched sleepily the flakes, silver and dark, falling obliquely against the lamplight. The time had come for him to set out on his journey westward. Yes, the newspapers were right: snow was general all over Ireland. It was falling on every part of the dark central plain, on the treeless hills, falling softly upon the Bog of Allen and, farther westward, softly falling into the dark mutinous Shannon waves. It was falling, too, upon every part of the lonely churchyard on the hill where Michael Furey lay buried. It lay thickly drifted on the crooked crosses and headstones, on the spears of the little gate, on the barren thorns. His soul swooned slowly as he heard the snow falling faintly through the universe and faintly falling, like the descent of their last end, upon all the living and the dead.