Cosmo's behaviour...any explanations?

Okay. Cosmo lOVES his tickles a lot.He especially likes vigorous ear rubs, where he plonks his head inbetween your legs and you rub his ears and neck... He moans.....
If I get on the floor with him he still wants to do it...sometimes he does tipply overs...other times he gets himself so his head is on my chest and I can give him an ear rub, but then he will start to dig he does it a lot now...(just to clarify I am sitting on the floor and he is between my legs!!!! )
So why the digging then???? He doesn't do it at any other point (only when he's being norty in the garden! ) nor does he do it with anyone else....???
Ideas?

Could it be you're scratching a place that makes his legs go?

Like when you scratch their belly and you make the back leg go like it's scratching?

It sounds like he's enjoying it anyway

He's definitely enjoying it!!! Muppet that he is.
No..it is a definite dig....on purpose...