Were they both females? Both males, opposites? Could be one was sick. Hard to say. Did she check on them before feeding? Maybe they had run out of food/water and turned on each other.

At my pet store they have a breeding pair of hamsters who have been together for a long time and had several litters and they came in to find the female with a huge hole chewed in her side and the babies nipped here and there. Babies were already a couple weeks old and not a spot on the male but it had to be him. Female was still alive though, super gross. I thought that was pretty bizarre.