I have never met a person who slaps a person on the back of the head as being anything other than an absolute prick.
It's passive aggressive behaviour. Smacking someone on the back of the head is a snide way of asserting dominance, the person is testing boundaries or letting you know they are comfortable enough to try it on.
Had a boss in work who used to do it and get his wedding ring to make contact.... absolute prick. I was early 20s - he did it to me and I lost my shit - the shock and sharp noise/impact... 6ft4 big bloke, I marched towards him like my legs didn't feel anything - pure jelly, hands felt like they weren't mine, I then stopped and walked away and told him "do not follow me" in a high pitched voice with tears in my eyes.... he started laughing and acted surprised as though I'd over reacted.
The worst thing was there was girls about.... the stupid fucker probably thought the fact that my voice went high pitched and I was almost crying and shaking meant I was upset. If I get like that it means my body is surging with adrenaline and things could get heavy. The man child did not have a clue.
He started to walk after me when one of the girls stopped him and said "are you stupid?!" - she knew the signs he didn't.
Humans aren't meant to react well to their heads being hit not least the back. It's a threat.
I hate violence, but I can't stand people who make me feel like he did.