What are my cat's neck spasms?

I have a cat, almost 10 years old, who in the last week or so has developed strange neck spasms. Basically, out of nowhere, his neck swings violently to the right, pretty much all the way around (unless there's an obstacle in the way). It's not a natural movement - his head swings in a way that suggests it's 'being swung', rather than him just wanting to look around. I'm pretty sure it's involuntary.

The first time I noticed it, he was lying on my lap, and he just whacked my leg with the side of his head. Since then, I've noticed it increasingly while he's upright - usually when he's lying down and relaxed (but, being a cat and all, that's pretty well 'most of the time'). He's hit quite a few objects doing it. When he's on my lap, it kind of freaks him out and he tries to run for a second. But it's just one at a time, and then he's good for a while. (I couldn't really estimate how often it happens, but I haven't noticed them any more frequently than a couple times in an hour.)

I've never seen him do anything like this before, and I'm wondering if it's anything to be concerned about.