Heading into the 2016 season, there was a concern about whether the Denver Broncos could sustain their level of defense from 2015. Using our Net Expected Points (NEP) metric, the Broncos were the top defense by a wide margin last season. By Adjusted Defensive NEP per play, which is adjusted for strength of schedule, Denver was more than twice as good as the second-best defense in the league. We all know they rode that to a Super Bowl victory.

But having an elite-level defense is not something that typically carries over from year to year. Except the Broncos have managed to pull it off this season.

Through 15 weeks, Denver again has the top-ranked defense by Adjusted Defensive NEP per play. Except, unlike last season, the Broncos aren’t in the driver’s seat of the playoff race, and with two weeks remaining in the regular season, they're going to need to catch some breaks to make the postseason.

The Broncos sit at 8-6 with a 24.4 percent chance of making the playoffs, per our models. All of their 24.4 percent chance comes from getting in as the last wild card as they are two games behind the current No. 5 seed, the Kansas City Chiefs, and three behind the division-leading Oakland Raiders. Right now, the 9-5 Miami Dolphins have a grasp on the final Wild Card spot with a 50.4 percent chance of getting into the playoffs.

With Denver on the outside looking in, let's take a look at why an elite defense wasn't enough to carry the squad this season.

Standing Ground

One of the biggest changes for the Broncos' defense from last season to this year has been the ability to stop the run. By Adjusted Defensive Passing NEP per play, the pass defense is even better than it was last year, but the run defense has gotten significantly worse. In 2015, the Broncos ranked fifth in Adjusted Defensive Rushing NEP per play, but they have dropped all the way down to 27th in 2016.

Opponents are averaging almost a yard more per carry -- 3.3 to 4.2 -- this season than they were against the Broncos last year. With more yards gained on the ground on earlier downs, opponents are seeing more manageable third downs than they were in 2015.

Last year, only 19 percent of opposing third downs came with three or fewer yards to go. This year, that’s gone up to 27 percent. Here’s a look at how opposing third downs have changed by percentage from 2015 to 2016, using data from the Pro-Football-Reference play index.

While the Broncos aren’t allowing significantly more conversions -- they gone up from 32.4 percent to 34.2 percent -- the shorter third downs take away obvious passing situations that allow Von Miller and DeMarcus Ware go on an all-out attack on the quarterback. That’s when this Broncos defense becomes the most dangerous.

The defense is taking advantage of those 3rd-and-long situations, though. Denver has a better sack rate on third down this season than in 2015 -- 9.5 percent to 7.3 percent -- but only three of the Broncos’ 21 third-down sacks come with less than three yards to go, which has seen the biggest increase in play volume.

Little Things

That big drop has caused a lot of small changes that have turned games in the wrong direction for the Broncos.

First comes from time of possession. Last season the Broncos defense kept opposing offenses to the fifth-lowest time of possession per drive at 2:30. This year, though, they’re more in the middle of the pack, ranking 13th in opposing time of possession per drive at 2:38. Those eight seconds per drive take away time for a Broncos offense that’s still struggling to score points -- something we’ll get to in a bit.

It also leads to a few more yards per drive. While the Broncos still lead the league in that aspect, they’ve gone from allowing 22.5 yards allowed per drive to 25.5. And with the 10th-worst average starting field position for a defense, those few extra yards can sometimes mean the difference in getting into field goal range or punting.

Field goals allowed have been one of the biggest differences on defense. Last season, the Broncos were ninth in field goals allowed per drive, but this year they’re 21st. That swing is enough to change the outcome of some games. Take this past Sunday, when the Broncos held the New England Patriots to just one touchdown, but New England also added three field goals during the game.

Unimproved Offense

Then, of course, there’s the offense. Technically, the unit is better by Adjusted NEP per play, up to 26th overall from 28th last season. But that hasn’t been enough to help out the defense during the year. Trevor Siemian has been slightly below average as a starting quarterback this season, ranking 26th in Passing NEP per drop back among 38 quarterbacks who have dropped back at least 100 times.

Denver ranks 23rd in Adjusted Passing NEP per play, which also includes two underwhelming starts and one other injury replacement from Paxton Lynch. Like the defense, the worst part of the offense has been the running game, which ranks 28th by Adjusted Rushing NEP per play.

With an inability to move the ball at a consistent rate, the offense has continued to be an issue. There have only been four games when the defense has allowed more than 21 points, but the Broncos are 1-3 in those contests -- with the one win a 25-23 victory over the New Orleans Saints. There’s been just as many games when the Broncos have scored 16 points or less, and unsurprisingly the team is 0-4 in those games. The most recent was the loss to the Patriots, which reportedly led to an offense defense shouting match in the locker room after the game.

It’s hard to blame Aqib Talib for his frustration when the secondary is playing even better than it did last year, but the rest of the team has not kept up. Some of these declines were not too surprising, but it won’t make it any easier to accept for the Broncos when they’re likely missing out on the playoffs just a year after their Super Bowl win.