Anticipating casts

Working on basic casting with my dog and I know he knows the casts. But now he has started anticipating he casts. During a basic three handed/baseball casting drill with the dog sitting at the pitchers mound he has started to lift his butt and shift in which ever direction he thinks the cast is going to be. Sit-shift-sit, etc. I realize this is a violation of sit, but he's solid otherwise. Just during this drill that it occurs.

Any suggestions on how to fix this?

My attempt to fix it so far has been to sit him and praise for sitting, then instead of giving him a cast ill walk out and pet him, move him, etc. just so he dosent know if its a OB drill or a casting drill. Ill do a couple reps like this then a cast or two, repeat.