Why is my head going all Poltergeist on me Lately when sitting, my head will abruptly turn at weird angles very suddenly. I'm not always able to catch it to stop it before it happens; sometimes the movements are really bizarre ( my head moving straight back, leaving me looking at the ceiling if my eyes were open). 

Is this a common/known occurrence in meditation, or do I need to see a doctor?
The experience you describe is not a common experience but meditators do get very uncommon experiences due to sudden outburst of past Karma / Snakhara. When you have stopped creating new Karma though Vipassana sometimes deeply hidden karma suddenly pops up to give results. (Many times cited by S.N.Goenka in his lectures.)

Also if would be an idea to also see a doctor and perhaps a serious meditator too to rule out any medical condition just be on the safe side.