Redirected Feline Aggression

Hi Chris,
You've been such a help so far with my very aggressive cat. She's doing fine as an outdoor cat.
She still has some peculiar aggression behaviour which baffles me. She becomes aggressive towards me or anyone else while cleaning out her litter box. Or, more often, it's when I'm bringing the litter to the curb for garbage and she smells it in the bag. I think she thinks I'm another cat or something. I always have to be very careful when cleaning the litter and I always look around to make sure the cat's not around. She will come up and growl and try to bite and scratch.
Do you have any explanation for this? So far I'm managing by keeping the door open as a quick escape route but any suggestions would help.
Also, if there is another cat around she will redirect her aggression towards me as well. Any suggestions on this too?
Thanks so much
Tinku