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?

Crowded animals will fight, perhaps that was the reason, though I don't know what constitutes being crowded and the behavior isn't well documented - the best i could find is a sentence in this DF wiki article.