The bed moves and wakes me up three times last night

Last night I felt the bed shake about three times, each in separate incidents. However, shaking is not the best verb to describe the movement.

It's like laying on your back, with your knee up. As you fall asleep, your knee will start to fall. You'll feel it falling and jerk the knee back up to the vertical position. This action creates a lateral movement of the bed, and that's what I felt.

Each time I felt it, I checked if Holly was the cause. Nope, she was on her stomach. No way to cause the movement. So other than someone (or something) standing at the side of the bed, giving it a little shove, I have no explanation as to it's cause.