I hope you didn't leave them in a hot car for an hour. Feeders or not, they still deserve that respect. No offense intended, that's just what stood out about this to me, and that could have stressed them enough for one to kill the other.

I've only had my little mouse colony for a couple months, but haven't had any deaths that I'm aware of. Females are separated out once they're preggers to have their litters and raise them, then they're put into communal groups for a few girls' nights out and a break before it's back to the dating scene. Males are always kept lone unless in with a female, since they tend to bicker constantly.