My cats are crazy

So, I am minding my own business, working on my questions for this week, when all of the sudden I hear this screaming noise! Well, I know it isn't me screaming, and the only other creatures in the house are the cats. So, I run downstairs and see that there are two cats outside looking into the windows - which is driving my cats absolutely crazy! I go out and shoo them away and think things are just fine, but when I go upstairs Seven has her tail all puffed out and Stupe is growling at her! So I think that this will all settle down and they are just freaked by the other cats, but they start trying to kill each other and I hear the same screaming sound. OK, now I understand being territorial with the other cats and trying to kill them, but each other? They have lived together for 2 years! Stupe even grew up with Seven. So, of course I think it is Seven that is being the instigator, but now I know the truth. I picked up Seven and calmed her down, but Stupe remains under the futon growling. I try to pick her up - and she growls and hisses AT ME! What the hell is with that? I am not a cat! I am her master! What the heck is going on with my cats? How do I get them back to normal? What weirdos.