As your question suggests, the cause of Louise Mallard's death in Kate Chopin's "The Story of an Hour" is forever debatable, but there are details in the narrative that can lead us to infer a cause. We know, for example, that
Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death.
The death of her husband, Brently Mallard, as one would expect, is a shocking event, and Mrs. Mallard's sister uses "veiled hints that revealed in half concealing" in order to disclose his death and to preserve Mrs. Mallard's equilibrium to the extent possible. At this point, we have no reason to think that Mrs. Mallard is anything other than a conventional middle-class woman in the late nineteenth century. We expect the news of the death of her husband to be the devastating news that it appears to be.
The first sign that Mrs. Mallard may be slightly unusual is in her first response to the news:
She did not hear the story as many women have heard the same, with a paralyzed inability to accept its significance. She wept at once, with sudden, wild abandonment, in her sister's arms. When the storm of grief had spent itself she went away to her room alone.
The important part of this description is "with sudden, wild abandonment," a phrase that doesn't quite comport with the picture of a woman in her class and with a heart condition. In addition, anyone who has just received such news would most likely seek comfort from loved ones close by—Mrs. Mallard's sister, for example. Instead, we get the sense that once "the storm of grief" is over, Mrs. Mallard's psychological (and physical) condition is much less dangerous than we would expect.
As she sits and thinks about her situation, Mrs. Mallard begins to feel something—"reaching toward her through the sounds, the scents, the color that filled the air"—that she tries to fight without success: the realization that the death of her husband, rather than ending her comfortable life, has opened up a life of marriage repression to unlooked for but welcome independence:
There would be no one to live for during those coming years; she would live for herself. There would be no powerful will bending hers in that blind persistence with which men and women believe they have a right to impose a private will upon a fellow-creature. A kind intention or a cruel intention made the act seem no less a crime as she looked upon it in that brief moment of illumination.
This is Mrs. Mallard's declaration of independence—not from an evil, abusive husband but from an institution that is, in her view, repressive even when the man and woman are "happily" married. The institution of marriage, not the partner, is inherently restrictive because each person's will is subservient to another's will. Mrs. Mallard's mantra is now "Free! Body and soul...
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free!"
Mrs. Mallard's explosive initial response to the news of her husband's death and her quick recovery before going upstairs foreshadow the overwhelming realization that her husband's death has freed her from a form of repression—no matter how outwardly pleasant—that men and women live with without questioning. This unexpected release from living for someone else insures a life of spiritual and psychological freedom that Mrs. Mallard has never even entertained as a possibility.
Unfortunately, when Brently Mallard walks through the door, having been incorrectly listed as "killed," Mrs. Mallard's heart stops, not because she is happy but because she sees her freedom crushed by the walking embodiment of the repression she has briefly escaped.
At the end of Kate Chopin's short story, "The Story of an Hour," the doctors say that Mrs. Mallard died from a heart attack caused by the "joy" of seeing her husband alive. She was known to have "heart trouble." He had been presumed killed in a "railroad disaster." Upon hearing the news of his death, she immediately grieves for him, but she eventually comes to a very important realization after spending time alone. Instead of being depressed, she knows that for the first time in her life she is free.
In the 19th century, when the story was written, women were very much second class citizens ruled by their husbands. Much of Chopin's writing is about women trying to break away from this "repression." The awareness that she can now control her life brings overwhelming happiness. Chopin writes,
There would be no one to live for her during those coming years; she would live for herself. There would be no powerful will bending hers in that blind perspective with which men and women believe they have a right to impose a private will upon a fellow creature.
Now able to exact her own "will" she whispers to herself, "Free! Body and soul free!" She looks forward to making her own decisions, and a life she once thought "might be long" is spread out before and she "breathed a quick prayer that life might be long."
Thus, the shock that makes her heart stop isn't joy from seeing her husband alive, it is shock at losing her new found freedom which she was very much looking forward to experiencing.