Are you sure your not shaking the bed yourself? I sometimes jump violently when im beginning to fall asleep, sort of like a hypnic jerk caused by my apnea and it feels like the bed is shaking.

My boyfriend does that when he's falling asleep, too. Most of the time, he doesn't feel it. A couple of times, it is really violent and when I comment on it, he sometimes thought it was me doing it!

If it persists and keeps you from falling asleep, then you might want to see a doctor, but otherwise, don't worry about it.