After all the Ma-tsu dramas, my first thought was parasite... If the muscle spasms are a pain thing, then it's probably not what Ma-tsu had. She didn't even notice the thing. Although all my colubrids do a head jerky thing we call flirting over here when we pet them which involves leaning into the touch too. But sounds like that's not what he's doing?

Wish I could be more help!