That's a normal motion they make as the rodent moves down the length of their body into the stomach area. It's especially noticeable in a shallow tub because they can't lift their bodies way up in the air for some gravity assistance like they can in a tall tank environment.

The first time I fed Butterscotch a rabbit (the burm at the zoo)...I saw her doing that same thing well after I thought the rabbit had moved far enough down her body. I'd been bumping around in the snake house a bit and figured I'd really bothered her and she was trying to spit up her meal. I put everything down, turned out the light and left. Came back an hour or so later and there was no sign of the bunny...she kept it down after all.