Hi
Sensitive skin would be my guess too. I have an itchy boy (Wirehaired Vizsla) with ticklish areas - if we touch those he quivers then starts scratching. He'll also shake his ears after I've peered in them, whether I've done anything or not!
Mind you, none of that stops him coming for affection. We are just careful not to tickle him if he's feeling a bit sensitive, as I don't want to start an itch-scratch-itch cycle.
How old is your dog? Has he shown any signs of sore or irritated skin? Are there areas where you can fuss him that don't get that reaction? It might be useful to get more of an idea
where or
when it occurs to see if it might be part of a pattern. Although that said, mine is a shaker - he'll often shake his head after drinking, and will do a shimmy shake after he's been asleep for a while. It's his "Woo hoo hello world, I'm up!" signal
Kindness is the language which the deaf can hear and the blind can see ~Mark Twain