Shaking REM and movement
In short this can be due to:


past fabrications surfacing rapidly and creating uncommon experiences
perhaps less likely but a possibility is Piti which sometimes create vibrating and / or pulsating experiences


Also see: eye movement during meditation, I get rapid eye movement as soon as meditation starts

These experiences are neither good nor bad. Also do not get attached or averse to them. In additions concentrate on the task at hand than giving importance and distracting your self when there experiences do arise in your meditation session.