Yup, that's something I've noticed only as I've begun watching myself back on video lately.
I haven't specifically worked on alternating technique much, but if you had asked me a few months ago I would have told you with certainty that I alternate my picking fingers, combined with raking when moving back up the strings. Only recently have I noticed that this isn't actually the case.
It seems to be linked to speed for me. The faster I play, the more consistently my fingers will alternate. When I'm playing a constant 8th note pattern, the only break from alternating will be raking.
So yeah, it's a weird one, and I'll probably look into whether it's something I should work on, or whether it's "weird, but functional", since it seems to sort itself out when the part requires it.