Mack didn't suddenly become a bad coach at some point. He always knew football, and he was good at instructing it.
However, I'd think at a program like Texas, it's almost easier to build it up than it is to maintain it.
You can always figure, "Man, if we just had that one special kid. If we just had a little better practice. If S&C were a little different..." you'd make it over the mountain. Once you get there, you try to remember the intensity that got you there. You want to keep it going.
But we're all human. Sacrificing the sleep and the personal family moments starts to grate. You wonder if a little less would be OK. The human body can only handle so much, so standards start to slip. After a while, you're "burnt out". You let the magazines recruit for you. You let the position coaches pick your formations and plays. Players start arriving late for workouts, and just do what they feel like.
I have no doubt that, after resting and being away, Mack's ready to lead again. UNC can definitely take the next step with him.