Join Date: Mar 2010
Location: Ontario, Canada
I can only think of three possible reasons that would result in sudden aggression. The first is the one you suggested...redirected aggression to a stray cat. The second is territorial aggression, which usually starts to rear its head when the cats reach social maturity, at or around the age of 2 (not sure how old your cats are). The third possibility is medical. Certain illnesses can cause aggression; alternatively, cats who are in pain (even from something like dental pain) can become aggressive. Perhaps there are other possible explanations, but none that I can think of. Whatever the reason, this sort of thing could have just as easily happened when you were out shopping for the afternoon, so don't blame yourself for leaving them overnight.
I would tend to first rule out medical causes for any sudden changes in behavior. If the cause is not medical, then I would suggest you do a short re-introduction of the two cats in question...and it would be best to do so in the near term, before the aggression escalates any further.
Dogs have owners. Cats have staff.