Its hard to say, but yor Neices Cat could have had a blod clot or anuyerism on the brain, and her thrashing and activity could have been the recation to the pain that she was feeling.
Cats react differently to pain, some hide and some try to fight , and trash about.
We are sorry to hear that she passed away, now at lease whatever hurt her wont hurt her anymore.
And she will be there for your neice in The Rainbow Bridge waiting to take her to Paradise.
One Fine Day.