I'm afraid I couldn't tell you how it started. It's been all of us in here for as long as I can remember.

[Mostly true. He does have some inkling of what caused it, but the memories are buried deep somewhere and he'd rather it stayed that way.]

As for triggers... Owen seems to have a very specific one, which is why he doesn't come out at all normally. With the rest of us, though... Well, if something happens that one of us can't deal with, that can cause somebody else to take over. It depends on the situation, really.