Which details from "The Legend of Sleepy Hollow" support the idea that the "headless horseman" was a trick to scare Ichabod away?

Ichabod Crane was well known in the community to believe in witches and goblins and the story of the Headless Horseman. He had a copy of Cotton Mather's book on witchcraft and:

His appetite for the marvellous, and his powers of digesting it, were equally extraordinary; and both had been increased by his residence in this spellbound region.

No doubt Brom Bones knew about his wild imagination and figured he could pull off a great prank and get rid of his rival for Katrina's hand in marriage.

There is a lot of foreshadowing about Brom Bones and his skill as a horseman, plus Brom told the story of having escaped the Headless Horseman himself, so he was familiar with the particulars of the legend and how to pull off such a prank.

He was famed for great knowledge and skill in horsemanship, being as dexterous on horseback as a Tartar.

Also, Brom was a big jokester, so it makes sense he would pull a prank on Ichabod:

He was always ready for either a fight or a frolic; but had more mischief than ill-will in his composition; and, with all his overbearing roughness, there was a strong dash of waggish good humor at bottom. 

and

He was, in fact, noted for preferring vicious animals, given to all kinds of tricks, which kept the rider in constant risk of his neck, for he held a tractable well-broken horse as unworthy of a lad of spirit

When Ichabod leaves the Van Tassel farm dejected because Katrina has spurned his advances, he sees the Headless Horseman at the very same bridge where Brom told his story of escaping the Horseman:

He affirmed that, on returning one night from the neighboring village of Sing Sing, he had been overtaken by this midnight trooper; that he had offered to race with him for a bowl of punch, and should have won it too, for Dare-devil beat the goblin horse all hollow, but, just as they came to the church bridge, the Hessian bolted, and vanished in a flash of fire.

When Ichabod is chased by the ghost, he says:

He recollected the place where Brom Bones’s ghostly competitor had disappeared. “If I can but reach that bridge,” thought Ichabod, “I am safe.”

Finally, after Ichabod disappears, whenever the story comes up, Brom Bones knows more than he is telling:

Brom Bones too, who shortly after his rival’s disappearance conducted the blooming Katrina in triumph to the altar, was observed to look exceedingly knowing whenever the story of Ichabod was related, and always burst into a hearty laugh at the mention of the pumpkin; which led some to suspect that he knew more about the matter than he chose to tell.

What evidence suggests the "headless horseman" is a trick to scare Ichabod in "The Legend of Sleepy Hollow"?

In Washington Irving's delightful ghost story, "The Legend of Sleepy Hollow," Ichabod Crane has designs upon the daughter of very wealthy Old Baltus Van Tassel, Katrina.  When he receives an invitation to partake in the "merrymaking" and "quilting frolic" at the Van Tassel castle, Ichabod is ecstatic.  However, Brom Bones, a suitor of Katrina's also attends. Since he and Ichabod are rivals for Katrina's affections, and he has already played practical jokes upon Crane.  Therefore, he may well have tricked Crane after he left.  Here are points consideration: 

1. When Ichabod departs from the party it is the "witching time of night" in the legendary haunted region of Sleepy Hollow.  As Ichabod rides along,

All the stories of ghosts and goblins that he had heard in the afternoon, now came crowding upon his recollection

and, as he approaches the stream that is believed haunted by the ghost of Major Andre, who had been taken prisoner in this area, Ichabod becomes unnerved.  He kicks his old horse Gunpowder so it will trot quickly across the dreaded bridge, but the horse runs broadside into a fence, instead.  Then, Ichabod urges old Gunpowder forward, but the horse balks.  Crane hears something and beholds a black, towering, misshapen form.   However, whether his perceptions are accurate is not certain because Ichabod has been frightened by all the ghosts stories that the old wives have shared throughout the evening, and he is at what is considered a haunted bridge.

2. After Gunpowder is found, calmly munching grass in the front yard of his owner, van Tipper, there is no trace of Ichabod Crane.  After much searching,

on the bank of a broad part of the brook, where the water ran deep and blck, was found the hat of the unfortunate Ichabod, and close beside it a shattered pumpkin.

This pumpkin could well be what Ichabod has believed to be the head of the headless horseman.

3. After Ichabod Crane disappears from the area, Brom Bones marries Katrina Van Tassel.  And, whenever the story of Ichabod is told, 

he always burst into a hearty laugh at the mention of the pumpkin; which led some to suspect that he knew more about the matter than he chose to tell. 

Of course, Brom Bones is known for his horsemanship and he owns a black, spirited steed in Daredevil.  And, he has already been known for his practical jokes played upon Ichabod Crane.

In "The Legend of Sleepy Hollow," was the "headless horseman" a trick to scare Ichabod Crane?

Irving doesn't tell us whether Ichabod Crane was a victim of a real "Galloping Hessian" or whether it was a prank pulled on him. From the comic way Crane and his behavior are described, we can presume that it was a prank by Brom Bones.

At the Van Tassels' party, people were telling ghost stories, especially about the Headless Horseman. Brom has his own story to tell about how he dared the horseman to race and bet a bowl of punch for the winner. Brom says he "should have won it too, for Daredevil beat the goblin horse all hollow, but just as they came to the church bridge, the Hessian bolted, and vanished in a flash of fire." In the next to last paragraph, Irving gives us another hint that Brom Bones was the culprit:

Brom Bones, too, who, shortly after his rival's disappearance conducted the blooming Katrina in triumph to the altar, was observed to look exceedingly knowing whenever the story of Ichabod was related, and always burst into a hearty laugh at the mention of the pumpkin; which led some to suspect that he knew more about the matter than he chose to tell.

What terrifies Ichabod into making his horse flee in "The Legend of Sleepy Hollow"?

In "The Legend of Sleepy Hollow," Ichabod Crane is riding back home from the Van Tassels' farm when he sees a horseman "of large dimensions" mounted on a large, powerful black horse. Ichabod tries to ride on ahead of the dark horseman then to lag behind, but the horseman keeps pace with him. When they reach higher ground, Ichabod sees the huge figure outlined against the sky and notices that this is no ordinary rider:

Ichabod was horror-struck on perceiving that he was headless!—but his horror was still more increased on observing that the head, which should have rested on his shoulders, was carried before him on the pommel of his saddle!

This terrifying vision drives Ichabod to shower his horse with kicks and blows, goading him on to go as fast as he is able. The horse now seems to be possessed by a demon and gallops straight past the turning for Sleepy Hollow and toward the church. Ichabod struggles to stay on the horse and is finally dislodged when the horseman throws what appears to be his head directly at him. The next morning, a shattered pumpkin is found beside Ichabod's hat.

Even when utterly terrified, it is difficult for Ichabod to send his horse into full flight, and the mysterious rider is able to keep pace when he does so, since Ichabod's horse is so decrepit—though it has the remains of a fiery temper. The horse is described in some detail earlier in the story:

The animal he bestrode was a broken-down plow-horse, that had outlived almost everything but its viciousness. He was gaunt and shagged, with a ewe neck, and a head like a hammer; his rusty mane and tail were tangled and knotted with burs; one eye had lost its pupil, and was glaring and spectral, but the other had the gleam of a genuine devil in it.