Head shake- Mexican Black King ?

I'm new to owning colubrids, gorgeous all of them. I do own boas and pythons.

I acquired an adult male Mexican Black King today. Impressively docile/calm, he shed today, no issue.

When he was resting on my lap OR in his brand new tank at times. I noticed he sort of shakes his head slightly side to side, almost a bit jerky. The movement encompasses his head and a bit past his neck, maybe one or two inches. There is no visible contraction of muscles under the scales.

My impression is he does this when he feels surprised perhaps??

He doesn't always do this, mind you I've only owned him for less than a day. For the moment there appears to be no pattern or periodicity to this "behavior".

Can any experienced colubrid owners shed some light on this?

Thanks in advance.

Could be something benign. Such as courtship behavior, triangulation, over stimulation, etc. With it being "jerky", I would lean more toward courtship behavior or over stimulation.
OTOH It could be something neurological, or more serious (such as an adenovirus or other illness), which would require assessment, &/or testing, by a vet.
It is hard to say what it likely is, or may be, without a video (or, otherwise, seeing it).