It's a lot of things that add up.

We have a good budding NFL Franchise QB in Goff.
We have a star RB who can run and catch.
We have a much improved Oline.
We have lots of new weapons on Offense.
We have a whole new offensive system that is cleaver and unpredictable.
We have a new HC who has changed the culture and brought a modern NFL mindset, accountability, and placed high expectations on each player and the team as a whole.
We've stayed healthy.
We have some good vet player-leaders.
The quality and character of our players has stepped up a notch, while letting some borderline guys and just plain career knuckleheads go.

It's all this and probably a couple more things one of you will mention.