The anatomy of a defensive improvement can have many different roots, because defense is less predictive from year to year than offense is. Defense can be driven by high turnover rates. It can be improved by players, by new schemes, by better fits. New coaches can bring in all of the above. Player health can be a big portion of the improvement. There's just a lot of ways for a defense to improve or decline in one year.
All you need to know about defensive improvement in the NFL can be summed up in the New England Patriots. They were our 31st-ranked defense-adjusted value over average (DVOA) defense in 2017. They were 16th in 2018. And now they are No. 1.
2019: -32.1% (1)
2018: 0.4% (16)
Stephon Gilmore and J.C. Jackson started to emerge as a tandem last season. Jackson was on fire throughout the playoffs, and the Patriots allowed negative pass defense DVOA in four of their last five games. (Negative DVOA is good for defenses, because it means less scoring.)
The Patriots have simply upped the overall aggressiveness of their defense, bumping the blitz rate to 35.5% from 30.9%, and they feel they have the ability to play man-to-man and not get beat.
The only major addition this offseason was free agent Jamie Collins, who didn't even cost the team a compensation pick to bring back after wearing out his welcome in Cleveland. In fact, the Patriots lost their best 2018 pass-rusher, Trey Flowers, to free agency. It hasn't mattered. Collins leads the team in sacks, and former undrafted free agent defensive tackle Adam Butler is tied for second. The Patriots and Baltimore Ravens look to be trailblazers in several ways in the current NFL -- chief among those is getting more and more aggressive with more and better quality defensive backs.
Looking ahead: New England's defense hasn't shown any real signs of slowing down yet. The Patriots even made some solid in-game adjustments on Lamar Jackson's Ravens.