Wet the bed - why?

I adopted what is probably a Cairn Terrier. He's nice, and all was well. He's about a year old and a street find. I've had him for a few months.

Last night he slept on my shoulder for awhile, and then I got up to do some work in the middle of the night (insomnia). He wet my bed. He's never done that before. I was just a few feet away at my desk.

Why? Punishment for me? Fear? Something else? Should I lock him in the crate at night? (He will bark.) He was fine before this and would sleep next to the bed. Sometimes he would jump onto the bed to sleep while I was asleep, but he never did anything like this.

Thanks.

-A