Allowing the Cats Outside = Epic Fail

Yesterday, the obscene heat here in the Bay area finally broke. I celebrated by eating an enormous serving of French toast with homemade strawberry jam at Della Fattoria and then lounging about on my back porch, reading. So while I read on the porch, I left the door open and let the cats out -- an experiment to see if they're ready (and I'm ready) to let them out more often. They spent the better part of a couple hours poking around the back yard: chewing grass, rolling around in the wood-chipped flower bed, lounging in the warm sun. All seemed well. After a while, feeling secure about their safety out there, I went inside to do a couple chores. 5 minutes later, the cats came TEARING back into the house, Scout chasing Violet, hissing and growling. Scout actually bared her teeth at Vi and tried to take a chunk out of her ear. I mean, they play-tussle all the time, but I've never seen them like this before. At first, I was worried that one of them had somehow gotten hurt, but Vi was fine, and when Scout finally let me near her (about 30 minutes later), she appeared to be as well. I have no idea what happened out there, but Scout is currently doing a great impression of a shell-shocked Vietnam War Vet. She's been hiding under the bed, and if she could mumble and twitch and talk to herself, I think she would be doing just that. As of this morning, nearly a full day later, she is still crazy skittish hisses and growls when Vi gets within 5 feet of her. What the HELL?