I agree with many posters that having a weak military can do it, but the other night I was minding my own business building up my island and preparing for war when my best friend Monty (~ +10 IIRC, shared religions, giving him techs/resources/etc) decided to declare war on me and drop 2 galleys with 2 Jaguar, an Archer, & a Chariot onto my island.
I was able to kill them the turn they landed and my counterattack took his entire island and left him with one city on a far away island. I know it's Monty, but I'm still befuddled why he decided to attack me.