There are tons of shortenings like that, which didn't begin as derisive names, but were declared "slurs" retroactively by the usual suspects.
Sometimes, sure, but Jap, Nip, and Gook, have a pretty obvious natural origin in WWII, Korea, and Vietnam. War tends to show off just how good we, as a species, are at inventing creative new slurs, but also at infusing previously neutral terms with mortal hatred.
Of course, the Imperial Japanese are a bit of a special case, since their own actions (especially in China) might have turned the term into a slur without any need for global conflict. Seeing the white-coated ghouls of Unit 731 in action forcing innocent men and women to rape each other at gunpoint for later use in syphillis vivisection experiments, or watching the hordes of wild dogs in human skin descend on Nanking in the winter of 1937-1938, would have probably made basically anyone forget their country's fascinating history and rich culture in favor of calling them things a hell of a lot worse than Nippy little Japs.
Inhuman acts, in general, get people treated as less than human. War, genocide, and pissing in the swimming pool are all fertile breeding grounds for clever new insults that target all kinds of indelible traits.
