Wiktionary
whydunit
n. A type of detective story in which the focus is not on who commit the crime, but what were their motives for committing it.
Usage examples of "whydunit".
An unusual whydunit in the rare literal use of the term, this novel takes an unsentimental and ultimately unsettling look at family dynamics.
Disappointingly, the tantalizing whodunit is discarded before the one-third point, leaving only a psychological whydunit that might have been more intriguing before the market was flooded with serial killers, but consummate novelist Rendell can always keep you reading.
Stout's stories are always great mysteries – whodunits, howdunits, whydunits – and they zip along at a pace that would leave the Great Man anoxic.