Sometimes it’s reluctance tinged duty. Like "I really like this community, it’s well moderated and the current mod team are pretty balanced in their decisions. Except they’ve said they’re struggling with the load. Oh no, if it becomes too arduous, we’ll start losing mods and then we’re at risk of losing the culture we’ve built up here if newer mods have different priorities.
Sometimes chipping in as a mod is the lazy option when the alternative is a worse browsing experience (or finding a new place when a community goes to hell)
Like you say, it’s a misplaced between altruism and my own motives