Its not just this year:

A larger-than-expected number of make-goods so far is stoking concern that the NFL may be in trouble. Last year, one buyer purchased 100 NFL ad spots and got six free commercials to make up for ratings that fell short of estimates. This year, that same buyer acquired 100 ad units and expects to get about 10 make-goods.
The decline in NFL viewership raises new questions about the invincibility of live sports at a time of great anxiety for the media industry, marked by a loss of subscribers to conventional cable and satellite services, falling broadcast-TV ratings and fierce competition from digital outlets. And National Football League games aren’t alone. The Olympics on NBC and the U.S. Open tennis tournament on ESPN also drew fewer viewers than previous years.
https://www.bloomberg.com/news/artic...rship-declines

Short version? NFL not immune to effects that are draining viewers from traditional TV.