Actually, any /nick command (search without the /) could be the culprit. Perhaps you have a script that maintains your nick, but the nick it's maintaining isn't your primary nick, so after you join the network, it changes your nick and it just happens to do it when you're joining a channel. Of course, if it does it everytime you join a channel (even after it's done it once), then it's some other cause. Still, "nick" is a good thing to look for.
