Temperature.
Just trying to think outside the box here, but does anyone know where this aircraft was parked during its grounding earlier in the year?
If it was in one, single location, it may have been subjected to three months of regular heat/cooling cycles which may have had a detrimental effect on the folded cathodes inside one of the cells, especially if they were still switched on during the period.
Yes, I know that this happens to most aircraft on a daily basis but we need to look for something which sets this particular device apart from all the others and a regular very hot days followed by very cold nights cycle may have played some part in degrading the battery.