I think that Norv didn't adjust his offense to the changing personnel on the Chargers. It seemed like every year gm AJ Smith was letting go of more and more talent without replacing said talent. He also whiffed on the draft a bunch of times after starting out great. Norv didn't take into account that the Chargers no longer had prime LT or Vincent Jackson or Sproles and wouldn't change up his playcalling style. The crazy part is I remember one year Rivers completed passes to a bunch of different receivers that they signed off the street basically and still somehow kept them in games. The offensive line kept getting worse as well since they no longer had left tackle Marcus McNeil or Kris Dielman, which was giving Rivers less and less time in the pocket, leading to increased interceptions and more sacks. McCoy and Ken Whisenhunt came in and built the offense around shorter throws and increasing time of possession to make up for the lack of talent on the offensive line. They did this in favor of chasing the big plays down the field, which Norv loved to do, and went with more conservative playcalling (sometimes too conservative). They also gave Rivers more control over changing the playcalls and making adjustments Peyton Manning style, which works in his favor.
I think the new gm Tom Telesco has done an amazing job bringing in more talent as well, considering how limited the salary cap is. He also isn't stubborn in admitting his mistakes, such as when he cut Derek Cox after just one year.The defense has gotten a lot better which takes pressure off of Rivers. Brought in Keenan Allen and right tackle DJ Fluker through the draft which has made a world of a difference. Danny Woodhead also was a huge signing but got hurt and is out for the season. Overall I think it's the system that he's in that is allowing him to flourish, along with more talent on the team.