Cause of dead cell on a flooded lead acid 12v deep cycle battery
The battery failure you can cause by abuse, so long periods resting with low voltage, or deep discharge, or over voltage, tend to affect all cells equally. So I suspect a mechanical failure in one cell.

Sometimes things just break, or have been built badly. The most likely causes of a single cell going down is some contamination in the cell causing a short between the plates. A badly built separator could fail early. The plates are heterogeneous, some of the active mass could have shed and be shorting the plates.

With 2012 vintage, you're probably out of warranty.

People who **need* backup power will often use a series connection of individual 2v cells. If one goes down, then 10v is often better than none. When designing the battery, make sure all your critical loads are tolerant of a lower supply voltage.