[jhautz]

Its some sort of gas that is created when the chemistry in the cell goes bad. Dont ask me what all that is cuz I have no idea.

Cells can swell from any number of things. Physical damage, over charging, over discharging, putting more load on them than they are rated for, etc. It could be that yours went below the minumum voltage from sitting to long without getting any charge. A year is a pretty long time to not touch it. Was it charged when you stored it?