This would seemingly be the most straightforward reason. The system flagged a copyright film clip. Though this doesn't explain why you didn't get a notice about just that video.
But honestly, why spend time worrying about this? It sounds like you barely used your account, so just get a new one.
The private video did not show the movie and showed my son getting excited and trying to imitate the cool beans bit. It was barely audible in a few seconds of the 20 second video and my son was talking over it so I doubt automated solution would likely not fingerprint it. It’s also about 10 years old and has always been private.
Honestly, you're wasting way too much mental energy on this. Youtube is a blackbox. Effectively, it bans and otherwise penalizes accounts arbitrarily with absolutely no recourse. You either use their services understanding this, or don't. It's pretty straightforward.
I don't know if that's a thing with YouTube and private videos, but when I got a copyright strike on my YouTube account because of a private video I never got explicitly notified about it.
I just have random stuff on my channel, similar to what OP describes and largely use YouTube for watching content. The video in question was a private video of me doing a quick demo of some project where a radio was faintly playing in the background (I didn't even notice that at first - only after finding the original video file and cranking up the volume did I realize that the mic picked up some recognizable music in the background).
I would have never noticed that I had a copyright strike if I wasn't bored and randomly clicking through the YouTube UI one day.