Battery expanded from charging
Mine did the same thing. Still works just fine. I think it was a little bowed out from the very beginning. It's about 14 months old...never been discharged. Last winter as well as right now, it's on a maintainer. Both times the car was driven before storage so the battery had a full charge before being placed on the maintainer.
If the battery does crap out, just cut the bottom out of it, gut it, and install a smaller battery inside the case.
If you decide to buy a new one, look up Fred Gilmore. He has the best prices I've seen.($219 shipped 14 months ago)