It happens rarely but frequently enough to be notable, I guess? iirc it's the brain checking whether your muscles have been paralyzed like they should (so you don't move in reaction to a dream.)

Edit: Hypnic jerk - https://en.m.wikipedia.org/wiki/Hypnic_jerk "Hypnic jerks are common physiological phenomena. Around 70% of people experience them at least once in their lives with 10% experiencing them daily."