My guess is it's a manufacturing defect in the battery that would have been caught if it had been a high volume part. You can bet Boeing paid an unbelievable amount of money for each of these battery packs.
So? Even if that's the case, it wasn't caught, and there's no way to be sure the problem isn't widespread among these batteries.
It doesn't really seem like a safety issue though. For a system this big, I can thing of much worse things to go wrong.
What?
What!? These are emergency backup batteries, catching fire on an all-composite very large airliner. How bad should it be, before it's a 'safety issue'?
Edit:
The pictures of the failure are interesting. The construction of the case and the way they appear to have mounted the PCBs on pillars is crude. Though the containment of the failure looks good.
What struck me was that the lid is just a bit of folded metal, held down with two screws per edge. There's no attempt at any kind of accident gas containment and venting to somewhere safe. There are dribbles of the vile 'stuff' down the outside of the case...
The control/monitoring PCBs are in the same space as the batteries. So one battery goes, so does _everything_.
The small gauge white wires (that appear to come from each battery terminal) are bundled together, and terminate in small plugs to the PCBs. Any short in these wires/plugs will place shorts across the batteries.
The whole thing looks like a design by someone with the "nothing can possiblie go wrong!" mentality.