Biome shifting when Bukkit restarts?

Hey,

I have noticed some strange Biome shift ("seasons" so to say) on my bukkit server.

When I started, my base was in a normal, green Biome (I guess the middle green, not dark green), with a few desert areas nearby.
Upon having to load a backup (server crashed) some trees in the vicinity started to get yellow, and I had some yellow-ish grass patches in my base.
The server crashed again, now it's snowing on the neighbooring Islands (which before had been green, too), and my base is completely autumn-colored.
A few weeks later, another server crash, and after this one, my base is now deep in winter. It is snowing on the entire Island.

I did not use a backup for all crashes, a couple of times the save seemed to be undamaged (got a few errors on first start, which vanished the next restart).

I wonder what happened? I do not have a seasons plugin installed, yet it seems I have seasons. Is the Biome somehow saved apart from the normal map info?

It seems possible that parts of the save got corrupted in one of the crashes, leading to a Biome shift. If so, is there any way to revert it (I like my Islands warm ).