“Defenses have evolved to these high-powered offenses and they’re saying, ‘Look we’re going to devise a scheme that’s going to allow you to go up and down the field,’” Kendle, vice president of  museum and archives for the Pro Football Hall of Fame, told NBC News. “But then we’re going to shrink the field and hold you to a field goal or turn you over.”

Warren Sharp, an NFL analyst, had a similar take, writing in late October that defenses are doing better at limiting long plays and have received some help from the new interpretation and enforcement of rules.

One rule that has apparently hampered offenses is tight enforcement of the prohibition against offensive linemen getting a head start to block downfield on passing plays, according to Sharp.

For years, referees appeared to turn a blind eye to the infraction, especially when the offender had no direct impact on action around the receiver. But tighter enforcement of that rule can ruin plays that call for quarterbacks to make late run-or-pass decisions.

And while crediting defenses, Sharp also takes offenses to task for conservative play calling and poor pass protection.

“It’s easiest to point the blame to defenses and their adjustments. They’ve adapted the last two years and are playing a style of defense that has reduced big plays,” Sharp wrote. “But the offensive coordinators are not blameless themselves.”

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In a recent segment on ESPN, former NFL quarterback Dan Orlovsky pointed to a new defensive strategy, “simulated pressure,” meant to confuse offensive lines and quarterbacks.

Orlovsky showed clips of a Kansas City Chiefs-Denver Broncos game, including one play when the reigning champs brought seven defenders to the line of scrimmage, looking like they were bringing pressure right up the middle.

But when the ball was snapped, many of those defenders peeled back. That led to three Denver offensive linemen blocking no one in the middle of the field and allowed pressure from the outside, leading to a strip sack.

“That simulation of walking those people up at the line of scrimmage creates a lot of chaos for an offense,” said Orlovsky,  a well-regarded longtime analyst.

Aside from innovative defenses leading to this TD decline, another explanation could be the increasingly powerful legs of NFL place-kickers.