What could have happened is, you missed him thinking about moving out.
In the span of a few days or so there will always be a villager deep in thought considering moving out, and if you miss it and don't talk to them, they'll just go ahead and do it... maybe that's what went down? You missed the day he was thinking, and the day he finally moved? (days where they're moving are the hardest to spot, because I'm pretty sure, if you don't talk to your villagers, there's no way of knowing unless you catch them mid-act).