I think the only evidence we get is that she had some kind of heart disease, a weak heart, maybe an arrhythmia. Reading this story, I always thought it was possible that this meant something else. The text does not say that Mrs. Mallard was afflicted with heart trouble. It says that ‘it was known’ that she was afflicted with a heart trouble. This tells me two things: 1) This is just the impression that others have of Mrs. Mallard. It might not be true. 2) She is referred to as Mrs. Mallard, she is always spoken of passively. Clearly, the news of her husband’s death was a liberation. She now had a will, and a persona of her own. If everyone thought she had a heart problem, it might be a result of how she was treated by her husband. Maybe he treated her like a dependent, helpless wife, thus asserting his role as the dominant male. He, and possibly others, may have used the ‘heart problem’ to keep her in this subservient role.
She does die of shock at the end, so this doesn’t rule out a heart problem. But the fact that she dies at the news of her husband being alive (instead of dying upon news of his death) supports the possibility that she died at the loss of that passivity (that weakened state, which included the ‘heart problem’). That is to say her heart (and her mind) was the healthiest it’s ever been when she thinks her husband is dead.
If this analysis is correct, she was afflicted by mental (and maybe physical) weakness induced by her repressed role as Mr. Mallard's wife. And she died from a broken heart or shock.