Why are ghosts are tormenting Jacob's past in Water for Elephants?
I would actually disagree with the inference of this question. I don't think that Jacob is tormented by anything wrong that he has done in the past or some misdemeanour that haunts him with guilt. Let us remind ourselves of the situation that led to such a sudden change in Jacob's life. His parents are both tragically killed, and he finds out that in spite of their apparent wealth, thanks to his father's generosity and goodness, he had not insisted that his clients paid for his services. As a result of this, his parents had remortgaged the house to pay for Jacob's education, and now Jacob loses everything: his home, his parents, and his life. As he takes his final exam to become a vet, he is unable to finish because of the impact of this tragedy, and thus leaves his education and runs away from the situation.
This is something that would cast a shadow on anybody's life, and Jacob is no exception. However, bit by bit, during the course of the story, it is clear that Jacob finds healing through being able to work as a vet and through his relationship with Marlena. Let us remember that the elderly Jacob who narrates this story is a character who has enjoyed a long and happy life. Therefore I would argue that it is innacurate to state that Jacob is a character who is tormented by ghosts for past misdemeanours.