If you watch other horses together in pasture, they perform "mutual grooming". They'll stand next to each other facing opposite directions. One will start grooming, or gently nibbling, at the other horse's withers. Then the groomed horse will begin nibbling or grooming at the first horse's withers. They'll generally groom along the withers and neck, and sometimes onto the back. They are gentle and it is a herd behavior that promotes closer ties in the herd. And it feels good to them.
So sometimes while a human is grooming her horse, especially if the human is using a curry comb or scratching with her fingers, her horse will turn around and try to groom her back. I don't allow my horses to groom me - I'm not a horse and don't want to be treated like a horse. Once you allow your horse to treat you like a horse, you open yourself to being on the receiving end of dominance behaviors like biting and kicking as well as comfort behaviors like grooming.
I keep my horses tied when they're being groomed, and if one does reach around to groom me, I gently push their head away from me. I don't hit them, slap them, or make a big deal of it. I either use my open hand on their cheek to push their head away, or I tug on their halter or lead rope to push their head away. After a few times, most horses understand that you don't want to be groomed and they leave you alone.
Your horse will still enjoy the scratching and grooming, though, and that is a great way to help him relax with you, trust you and bond with you. So enjoy your grooming time - without being mutually groomed!