It was kind of weird. There were no health reasons for why he'd need to regenerate. I kind of assumed it was something that maybe the TARDIS decided to do. He chose to be the Warrior because he needed to be someone capable of doing horrible things. With the war over, he no longer needed to exist in that fashion, and so the TARDIS helped him return back into the man he was.
Or something.
Actually, this makes perfect sense. Or at least it would have done if this version of the Doctor had not already redeemed himself. In fact, I didn't see anything bad or wrong about him in the first place? He seemed a perfectly reasonable kind of guy. And in the end he didn't even destroy Gallifrey so had no burden or cross to bear.
He could have at least been taken out by a falling grand piano.... :/