The cross-flex rule, i guess.
I was kind of curious, since Fox had Thom Brennaman and Chris Spielman doing the game in Chicago, and the Chargers are a AFC team. I guess the NFL didn't want too many CBS games this week.
Since the Chargers were the road team, this game would have been on CBS per the old rules. I think all bets are off now.
From what I remember, CBS, and NBC before them, used to complain that the AFC had too many small markets compared to the NFC. They were right, of course. The NFC has 13 of its 16 teams in Top 20 TV markets, while the AFC has 8 of 16. #18 Orlando and #20 Sacramento do not have NFL teams, but are close to NFC markets. I'm willing to bet that this difference is why some NFC games are being switched to CBS, and a few CBS games go to Fox.
The NFC has NYC (1), LA (2), Chicago (3), Philly (4), DFW (5), Washington (6), San Francisco (8), Atlanta (10), Tampa/St. Pete (11), Phoenix (12), Seattle (13), Detroit (14), Mpls/St. Paul (15), Charlotte (23), New Orleans (50), and Green Bay (67).
The AFC has NYC (1), LA (2), Houston (7), Oakland (8), Boston (9), Miami (16), Denver (17), Cleveland (19), Pittsburgh (24), Baltimore (26), Nashville (27), Indianapolis (28), Kansas City (32), Cincinnati (35), Jacksonville (42), and Buffalo (52). Las Vegas (39) will replace Oakland (8) in 2021.