I've had it happen a couple of times, usually on a big post with pictures. I don't know what causes it. Also, sometimes in the middle of writing a post, the site will sign me out and not let me post until I sign back in.
Yea, that's what happened to me. I had written a post but when I went to submit it the site hung up. I couldn't even get back into the site for over an hour. It's fine now though.