He’d done so much for Hermione, for the Order, during the war only for it to be ignored and to be thrown in Azkaban for his "crimes". He thought it was going to drive him mad. Having both Draco and Theo being sentenced to life in that hell hole had pushed Hermione a bit over the edge, not that she had ever been a particularly light witch to begin with, even during school. Setting a professor on fire, locking a woman in a jar, setting up another woman to be abducted by centaurs.