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I am not aware of any concerns with neutering that could be the cause of this. Sometimes neutering can cause incontinence but this sounds to be completely unrelated.
I would see the vet for an exam and to have this checked.
The most common cause of this aggression around the tail is a flea allergy, even if you don't see fleas. It feels pretty good to them to have it scratched so they get a little excited when you do. To check for fleas, you should use a narrow tooth comb and comb through the hair over a white towel or paper towel. If you see black dirt fall out, that's flea dirt. If you wet it, it will look like smears of blood.
Keep in mind that cats can also have problems with their anal glands and this can cause a great deal of pain in their hind end. If you suspect this, your vet can express the glands and give him some relief.
Let me know if you still want to discuss this. I would be happy to tell you what I know.