It was what should have been a routine flag on the Washington Redskins for unnecessary roughness, one that gets called basically every single time around the league. A quarterback gives himself up by sliding feet first, concurrently, a defender leaves his feet to make the tackle and ends up hitting the quarterback. This particular case had the added exclamation point of a hit to the head.
Like most of the hits, they are not intentional. They happen quickly, and as stated above, the defender has left their feet and can’t control how he ends up making contact. But intent does not matter in the NFL.