War dogs fighting each other

I recently had two war dogs chained up next to each other, guarding the entrance of my fortress. Suddenly I noticed blood and body parts covering the hallway and, checking the reports, it turned out the war dogs had somehow turned on each other, one managing to kill the other.

I've never experienced this before - I was under the impressions dogs were automatically "domestic" and can't revert to a wild state. If that's the case, what triggered this fight? A bug or a feature?