This has happened to my alliance in the past. I’ve yet to verify if this is correct, but I think tier is based on Alliance rating specifically for alliances that have enlisted for the next war. That is, certain alliance take breaks at the ends of season or during parts of the off-season, which effectively shifts other alliances up a few spots. When they do re-enlist however, you’re similarly subject to getting bumped down due to being right cutoff of a tier.