ESPN, Fox, NBC and CBS are spending an average of $5 billion a year combined for NFL rights through 2021.

This season, network viewership is down about 10% from last season, according to Nielsen data, with steeper declines for prime-time games on Sunday, Monday and Thursday. 

“We’re scratching our heads,” said Andy Donchin, a media buyer at Amplifi US, an ad-buying unit of Dentsu Aegis Network, whose clients include General Motors Co. “We cannot pinpoint any specific reason why the numbers are down. It is probably being caused by a confluence of events.”

The election has been cited as a potential factor.

Rating declines for Sunday afternoon aren’t nearly as steep as the prime-time telecasts, but Fox and CBS are down, while cable news ratings are up.

“The election is probably the biggest factor that you could point to,” said Fox Sports President Eric Shanks.

Another factor has been Tom Brady on a four-game suspension, the retirement of Peyton Manning and injuries to Tony Romo and Adrian Peterson.

“We’re missing some stars out there,” said Brian Rolapp, the NFL’s head of media, who noted the NFL has gotten off to slow starts before “and [has] done just fine.”

ESPN has experienced the biggest drop, down 17% to an average 11.3 million viewers. Excluding the game up against the debate, ESPN is off 11%. NBC is down 13% from last season.

On Sunday afternoons, CBS and Fox are both down 3% in viewers.

“In all honesty, we don’t see any evidence of that,” Rolapp said when asked if some of the negative stories surrounding the NFL are hurting ratings. “We’ve been in the news in other ways before and haven’t seen a material impact on ratings.”