I don't think it was a medical issue. It was simply that he was coming back from a serious injury to his lower body. That affects you in a lot of ways. The previous year, Dak both strained his throwing shoulder and his right calf coming off his broken ankle. Neither of them was caused directly by his ankle injury but indirectly were the result of it. Flexibility, trust, etc., have to be reestablished after a serious injury and sometimes it just takes time. There are some players like Terrell Owens who could seemingly hop in a hyperbaric chamber after a devastating injury and return the next day ready to play and there are some who just need more time to get there even when the injury is fully healed.