Like everyone else I agree there is no one thing. It's a combination of great coaching hires/new scheme, Goff's development, Gurley's resurgence, a massive upgrade in WRs 1 through 3, etc.
But to me personnel-wise the OL is what probably surprises me the most. Let's remember just
how awful that unit was a year ago. I mean bad.... They made probably the biggest single upgrade of the NFL offseason going from Greg Robinson to Andrew Whitworth, they go from below average to above average at center. But 3 of the 5 starters from a year ago returned and at this point you have to call this one of the top 5 offensive lines in the NFL. That's pretty astonishing. I expected improvement, but more like going from bottom of the barrel to around average. The work the unit is doing in protecting Goff, opening holes for Gurley (and Austin and Brown) is just so impressive.
Also have to tip your hat to the blocking of the WRs downfield.
So much better. The TD to Woods on 3rd and 33 was partially made from good downfield blocks by both Kupp and Watkins.