That's an absurd way to train, much less treat, your franchise quarterback.
I cannot speak for his other fumbles, but both fumbles against the Cowboys were players ripping it out.
The imact on the ground had nothing to do with it and subjecting him to something that could cause injury (to his arms and shoulders no less) is something I hope the NFLPA sees and files a complaint about.
If they really wanted to help him, a better training method would be to set up a gauntlet run and have players try to strip the ball from him.