Hi Twiggy! You ask a good question...fortunately, I have several good answers!
1. If he stares intently at your behind afterwards....then he is getting off on the angry red blotch he lr=eft on the cheek.
2. If he nods approvingly at the angry red blotch, he is "marking you" as his.
3. If he gives you a severe look afterwards....then it has something to do with dinner.
4. Finally, and most dangerously....if you notice that whenever he does it..there "just happens to be" an optical sensor nearby, then he is using you for practice when he meets his friends at the bar to compete at "Spank the Booty":
https://www.80r.com/games/spank-the-booty
If you find yourself letting out a shrill sharp scream.....and living a life of fear....then you know he is getting good.
View attachment 2521418
GOOD LUCK!