| Having spent most of my working life with batteries I have seen hundreds just like yours. The distortion is either caused by overheating (just getting too damn hot - or overcharging which causes the battery to generate heat internally) or is the effect of positive plate growth. Positive plate growth is usually caused by incorrect charging. It can sometimes be caused by undercharging or over-discharging a battery and then recharging it at high rate. Occasionally it can be caused by physical damage to the plates (caused by vibration) which reduces the amount of active material. If you then charge at the"normal rate" the battery (because it now has less active material) is effectively overcharged.
I have purchased cheap South American and Chinese batteries that failed very quickly just beacuse they are badly built and distrort like this in normal use (caused by contaminated lead or electrolyte). Both my American and Chinese battery suppliers send me 5% more batteries than I order to cover "infant mortality"(ie batteries that are duff when I receive them).
I'd advise:
1) Check your charging voltage as mentioned above. 2) If you have an alarm that regularly drains the battery invest in a trickle charger. 3) If all checks out ok, buy a reputable replacement battery.  | 
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