Why do cats pull fur from there tails?

Cats groom themselves by licking and pulling at their fur with their teeth to remove anything stuck in their fur. If they are pulling out large patches of fur, resulting in bald spots, they could have a condition which requires treatment by a veterinarian.

maybe because she might have a skin disease or bad itch

I'm no vet, but I did have a cat doing this. It turned out to be a cancer in her belly fat that needed to be removed. Head to the vet.