I think the chapter title is "whydunnit", a play on the genre trope "whodunnit", which was a subgenre of detective fiction. Whereas whodunnit deals with "who" did the crime, a whydunnit fiction deals with "why" the crime was perpetrated. It's usually a similar puzzle-based narration with clues and suspects, but rather than trying to find the culprit, it usually challenges the readers to deduce the culprit's motives.