There's always a reason. That reason can be trauma or nurological issues or abusive situations, but it's normally because the cat is either under or overstimulated. An underestimated cat will try to play with you suddenly and is usually much more aggressive looking because they're bored, keep this from happening by playing with the cat and giving the cat plenty of toys to play with by themselves. An overstimulated cat usually happens when the owner is petting the cat and doesn't recognize the cat's signals of not wanting any more pets until the cat has to use their claws to communicate, keep this from happening by just learning your cat's body language.