. . . is apparently because so many of them have been arrested by cops - sometimes for repeated offenses.
If you want the short-and-simple version, USA Today has published a list of NFL player arrests going back to the year 2000, in reverse date order. It's very informative.
For the full, much longer version, a Web site provides searchable information on NFL arrests. It's been so overloaded in recent days that the primary site is currently offline; but it's been mirrored on a backup site, which is still working.
The Web site notes:
Keep in mind there are 1700 NFL Players and their arrest rates are lower than the USA arrest rate.
That's all very well, of course . . . but if you look at the NFL players who are actually taking a knee in protest, and compare their names to the arrest database, there appears to be a strong correlation - and that would make the arrest rate for the former a lot higher than the USA arrest rate.
Makes you think, doesn't it?