Could have been a flag on two fronts. The only question I see is if you consider the hit "unnecessary" per the rule wording below. The other frame of reference is that hit on Cooper by Ramsey the week before where it was initially called a fumble but reversed. If he didn't fumble that means he did not clearly become a runner. So perhaps that one was not deemed an unnecessary hit because I don't think you have to stand there and let him catch it.
Moderator Edit: Tweet removed per embedded text circumventing language filter (site rule 13).
ARTICLE 9. PLAYERS IN A DEFENSELESS POSTURE. It is a foul if a player initiates unnecessary contact against a player
who is in a defenseless posture.
(a) Players in a defenseless posture are:
(1) A player in the act of or just after throwing a pass (passing posture).
(2) A receiver running a pass route when the defender approaches from the side or behind. If the receiver becomes a blocker
or assumes a blocking posture, he is no longer a defenseless player.
(3) A receiver attempting to catch a pass who has not had time to clearly become a runner. If the player is capable of avoiding
or warding off the impending contact of an opponent, he is no longer a defenseless player.
(4) The intended receiver of a pass in the action during and immediately following an interception or potential interception. If
the player is capable of avoiding or warding off the impending contact of an opponent, he is no longer a defenseless
player.
Note: Violations of this provision will be enforced after the interception, and the intercepting team will maintain
possession.
(5) A runner already in the grasp of a tackler and whose forward progress has been stopped.
(6) A kickoff or punt returner attempting to field a kick in the air who has not had time to clearly become a runner. If the player
is capable of avoiding or warding off the impending contact of an opponent, he is no longer a defenseless player (for
enforcement, see 10-1-1 and AR 12.72).
(7) A player on the ground.
(8) A kicker/punter during the kick or during the return (Also see Article 8-h) for additional restrictions against a kicker/punter).
(9) A quarterback at any time after a change of possession (Also see Article 11-e) for additional restrictions against a
quarterback after a change of possession).
(10) A player who receives a “blindside” block when the path of the blocker is toward or parallel to his own end line.
(11) The offensive player who attempts a snap during a Field Goal attempt or a Try Kick. He is no longer a defenseless player
after he has had an opportunity to defend himself or moves downfield.