According to my Aunt, she (the cat) was at sleeping the foot of my Aunt & Uncle's bed and one of her (the cat's) legs started to flail, not unlike a dog getting scratched. That stopped after a bit, and the cat looked very surprised about it for a while. She went back to sleep, they started to go back to sleep, and then the entire cat started to flail. That kept up for a bit, and then nothing.
They waited a bit, called to her, poked her with a foot, then went to check her pulse and breathing. Nope.
We're guessing a stroke (which, for those wondering, gets its name from the idea that one was struck by an invisible weapon by a passing fairy. Stroke is short for "elf stroke". The more you know!), currently. Before putting her in a box and taking her downstairs to the basement, I opened her mouth and popped her tongue back in, since the tip was sticking out a bit; just because one is a dead cat, one doesn't really have to go to the grave looking like an idiot who can't keep her tongue in her mouth.