Hey R Aaron,
Personally, when my head "snaps back" its because I've nodded-off during the meditation

But there are other reasons why your head could be moving around unpredictably in an "energy jolt" sort of way. In the MCTB these sorts of phenomena are often discussed in the context of the A&P.
What are your meditations sessions like (painful, distracted, calm, full of visions, focused, etc.)?