Best genuinly funny reason:
Once, on a team fortress classic server where I was an admin myself, we installed a plugin that gave you credits for kills, flag captures etc. You could than buy all sorts of gimmicks with the credits.
All of the regulars, including the admins, immediatly started looking for ways to make easy credits. After a few days I thought of a brilliant little trick (I won't go into the details, that's not important here) to make credits a a rate way beyond anything anyone else came up with so far. So I was raking in the dough when all of a sudden *BAM* insta-ban out of nowhere. No reason given either.
About 30 mins later I was unbanned by the server's owner (a friend of mine). It turned out the credits plugin had a build-in system to automatically ban anyone making credits at a supposedly impossible rate. And here's the kicker: it didn't take 30 minutes because he didn't know yet. In fact, he got a notification right after the ban. The actual reason was that once he figured out what happened, he was laughing so hard he couldn't even sit in a chair.
Best not-so-funny reason:
I play a lot of left 4 dead 2 versus with random people. Enough said. Think of any reason, no matter how ridiculous, you could be kicked and I guarantee it has happened.
If you play l4d2 you probably know exactly what I'm talking about. If you don't let me give you the quick version: anyone in your team can at any time start a votekick against you. Not just admins. If two more people vote yes, you're out. The entire process usually takes about 5 seconds, you usually can't even say something in time. It's a good way to deal with griefers, but it's mostly used to kick people out of anger because your team is losing. On top of that people seem to automatically vote yes on pretty much anything. If you start a votekick completely at random against a guy who didn't do anything wrong you still have about 50% chance he gets kicked. Think of all the teammates who have ever been mad at you for some reason. Now imagine if they had had the power to kick you. That's the l4d2 problem in a nutshell.