[Al Skierkiewicz]

Daniel,
This is common failure for these batteries. In almost all cases, an inspection of the bottom of the battery will show some damage. It is likely that the battery was dropped at some point. The construction of these batteries reduce the gap between plates (compared to previous gel cell types) and that gap is filled with a wet glass mat. When dropped, the plates may move to close the gap. Eventually when charged, the charge voltage exceeds the breakdown provided by the gap and causes a short internal to that cell. When that occurs, temperature skyrockets in that cell, demonstrated by the swelling of a small section of the side of the battery. Temps rise so fast that the small gas valve cannot release all of the pressure. A combination of high temperature and excessive gas pressure causes the case to warp outward. When the battery cools, place it in two plastic bags to prevent the leakage from damaging other parts and hands and recycle as advised above.
Remember that each cell is rated at 18 AH and each cell is capable of providing 500+ amps when fully charged. That is easily 1000 watts per cell until the the energy is given up as heat and the cell discharges. The damage due to dropping will not always show up immediately but I have seen a few brand new batteries that have exhibited these issues.
These batteries can be used in any orientation and can be charged in any orientation. I personally only charge when battery is standing up to give the pressure valves a better chance of doing their job.
If you could supply a picture of the bottom of the battery, we might be able to see the cause.