Basic scenario is this: kids are tired. Very tired. On the way back home for snack & nap time, the big one bites the little one. She doesn't know why she did such a thing. Little one cries, big one shrugs her shoulders. Big one is told that there will be no snack, only bed. Chaos ensues.
Oh, and sorry for the shakiness. We were in Mark's car (obviously) and it has crappy suspension.