Screams of TERROR

Sometimes I really wonder what goes on in the mind of an almost 15 month old. OK, most times I wonder what goes on in the mind of my darling little almost 15 month old.
This weekend I was hosting a girls night at my house. Since the ladies were coming over I wanted to clean the house up so they didn't see the PIT that it had become. 3 cats, a baby, working all day and an hour and 15 minute commute (including taking AKL to the sitter) each way kinda slows down the whole 'keep the house clean process', but back to my darling girl.
I had the door propped open in the kitchen so that AKL could easily walk outside and jack around on our deck. YES we have a gate, have it blocked off so she can't fall - she'd need to be superbaby and have the ability to fly to get off that thing. I'm working on cleaning up the kitchen and AKL is just wandering inside, back outside on the deck, back inside. Well on one of her trips she walked in, walked over towards me, and screams a blood curdling scream. Seriously a 'someone is digging my kidney out with a spoon and taking my teddy bear to boot' scream. I of course flip around and look over at her thinking she's broken a bone, or any other horrible thing, and she just smiles :) So then that must have been WAY to fun because she proceeded to walk outside, scream for her very existence, walk back inside and repeat said process over and over again.
So tell me, what EXACTLY goes on in that little head to think that it's way to exciting to scream such a loud and 'someone is killing a baby' scream? I wondered what the neighbors would think. We don't kill babies!!