I think it's all mental with Zeke. He hasn't been fast since 2016, but he's still been more effective than this. PFF actually gave him their RB of the week award in week 1, so whatever physical shape he's in, it's not enough to stop him from being one of the best at his position.
My theory is that as the season went downhill, Zeke just mentally checked out. He's never been a motivated player in the best of times, so once his OL fell to pieces, once his defense took him out of games, once Dak got his ankle shredded, he just quit caring. And it's been mental mistake after mental mistake for a guy who made his bones on taking care of the little things.