If I play back the video camera, the dogs ignore each other's barks, but don't recognize their own. I think it sounds different to them, just like our voices sound different to us on tape. So if Cass, say, hears herself on tape, she thinks it's a strange dog and raises the alarm...and suddenly there are seven other dogs backing her up, even though they have no clue why she's barking.
This is one reason I have ear plugs handy when I'm editing tapes