I was cutting fabric on my worktable and Milo (age 9, neutered male) was sitting on the fabric. I nudged his hind left foot, the only part of him that was in my way. He reared up and slapped my head with his claws out, right-left-right-left. Both he and I were shocked. I said "Milo!" and he jumped down and ran out of the room. The rest of the day he repeatedly attacked his littermate Max (NM), biting his hind leg. Max ended up under the bed shivering. How could I have eased the situation? I've had the pair for almost a year and this has never happened before. Both are strictly indoors.
He may be trying to defend himself from you nudging his hand, and left foot. Sometime cats defend themselves when they feel like you were bothering them, it's completely normal for them to act like this, Milo might had evolved with that feature. Also if he's attacking his Littermate, he mighted had learned that from his ancestors. In this case take Max and Milo to the vet.