This is exactly it only its much deeper than that. Trump was the main owner back in the USFL, he owned the New Jersey Generals, and he is widely blamed for the USFL’s demise. He pushed for the league to play its game in the fall and directly compete with the NFL which is what ultimately hasted the USFL’s demise. As the league was getting ready to fold some teams merged including Trump’s General’s and the Houston team and on paper they had some pretty good superstars such as now Hall of Fame QB Jim Kelley. As the league was folding Trump tried to get the NFL to expand and accept his team and some others into the NFL but was told no.
So Trump was already angry at the NFL.
Then in 2014 he tried to buy the Buffalo Bills and got massive pushback from the league. He was then outbid.
So now he’s doubly angry and he holds grudges.
I think he was licking his chops trying to find a way to get back at the NFL. He saw how the initial anthem protests hurt the league’s popularity and was being blamed for an attendance decline. He was probably gleeful at it. Problem was that the whole thing was dying out and league wide last season it got to where there were only about 4 players still kneeling across the whole league and the media was no longer paying attention. So he decided to throw gas on the near dead issue by bringing it up again and now he keeps bringing it up everytime the league tries to put it behind them. It continues to damage the NFL brand which is exactly what Trump wants. Because he is a bitter, petty, manchild with grudge.