I think randykato is right. A .pdf from Visteon someone posted here shows the cooling system for the battery. It has a coolant loop. The A/C keeps the battery in a pretty darn tight temperature range. This is how they get 100,000 miles out of it. Its even possible for the engine to kick on when the cabin A/C isn't on in EV mode (so the compressor can cool the battery).

So.. It makes sense that the battery might have gotten out of its temperature range and the system shut itself down to protect itself.