I agree that what makes her crack is that Hamlet kills her Father. In her mad scene where she is passing out the flowers, she makes no mention of Hamlet, only her father. I do not take the purley romantic route that it is merely Hamlet pulling his affections that does it, other wise she would have gone just after the Nunnery scene, it is Hamlet killing her father.

Her madness is plausible to me, however for the following reasons.

She is a very young woman who has always been very reliant on the guidance of her menfolk: father, brother, and eventually, husband. All of these are ripped away from her in one way or another: first her brother, then her father (by the hand of, in her hopes) husband. Everything she knew and understood cracks. In the larger picture, I see her as a parallel to Hamlet (all the young folk are paralells to him in some way, I believe) - he contemplates suicide and says no out of fear. She faces death like the babe that she is.