He’s probably saying, “hey! I don’t want to go back in!” Foot stamping can be a bunny’s way of signalling danger; so it could be that maybe you startled him trying to pick him up, or it could just be his way of protest. I read that it’s good to let bunnies hop in to and out of their cages on their own free will as much as possible, that way they feel like the cage area is their home. If we put them into their cages they might interpret it as a punishment. What I try to do, is tempt my bunny back in with some veggies or a raisin treat, and they he goes in when I want him to, but he thinks that he’s making the decision. This took a while at first, but now it’s pretty easy–he often just zooms in (he loves veggies!). This way, the cage just becomes another happy place for him, and not taking away his freedom.
If it’s the kind of cage that he can’t hop in to on his own, maybe you could call him over with a treat and pet him for a second before picking him up, just so he can get used to the idea. We’re big on bribing here in my house.
Good luck with your beautiful boo!