First, regression doesn't mean that I expect it to continue to do so. Just like if I said he progressed that I mean I expect him to continue to do so. Saying he regressed means that he was a worse QB in 2015 than he was in 2014. That it is.
To why he regressed, none of us know. But don't forget that he was pretty ordinary in the playoffs during the 2014 season. He was lucky (no pun intended) enough to meet the forever imploding in the playoffs Bengals, the QB-less Broncos, and then they got crushed by the Pats. He had 3 TDs and 4 INTs, completing 58.5% of his passes in that playoff run. He also had an awful December in 2014 where he went on a three game stretch where he threw for 4 combined TDs and 5 combined INTs. So I can argue the regression started in 2014.
Also, you keep on harping on those 40 TDs, but 17 of those TDs came in four games against bad defense (Jax, Tenn, Giants, and WAS). I know it a little unfair to cherry pick stats like that, but you take away those four games and he threw 23 TDs and 14 INTs for the rest of the season. Not all that impressive. He had some stellar games against some awful defenses to make his stats look better than he played.