Why do monkeys through poop at each other?

Many animals defaecate when they are frightened as losing a little weight is an aid to running away faster.

Our cousin the monkey is probably up in a tree and is not in immediate danger so the faeces becomes a readily available weapon with which to annoy the threat.

Throwing dung is also seen in Homo Sapiens. If you've ever watched an episode of Lock-up or Lock-down which documents life in American prisons, It is not uncommon for inmates to hurl their dung at staff or each other. Often fecal matter is stored and mixed with urine then left to fester into a noxious brew and used on a victim at an opportune moment.

A2 yep