Ninety-nine percent of the time it is the latter, which raises the questions:

Why do monkeys throw their feces?

and more importantly...

Why do humans find it so damn entertaining?

Zoologists claim, that for monkeys, throwing feces is a defense mechanism.  They will do it when they feel threatened or angry.  There is a flaw to this widely accepted theory however, for studies have shown that monkeys will also resort to poo tossing when they are bored. Which means that throwing feces must be more than a mere defense mechanism. It must also be a crap load of fun! This has lead me to my own theory, which like all my theories, is rooted deeply in slapstick comedy.

Monkeys throwing feces is the primate equivalent of the pie in the face gag.

We all know that monkeys are all natural born entertainers. Monkeys don't have access to comedy devices such as pies, seltzer bottles, or Burt Reynolds, so like any great showman they use what is available to them- their poo.