Crying in your sleep?

OK, so this is a more serious topic seeing as it's... pretty serious looking in any case.

My girlfriend has said for a while now that she sometimes catches herself crying in her sleep. At first we decided that the answer to the question of 'why', was that is was her way of processing the things she experienced earlier.

Now, that sounded pretty woo-woo to me, and now I've actually seen her do it myself - which scared the hell out of me, by the way, it looked spooky, not at all what I thought it'd be - I've actually gotten quite concerned.

So what is it? Why does it happen?