Usually because it's a boundary thing, where the ball sailed way out of bounds. Or they decide their feet just got tangled up, or hand fighting on both sides, or whatever. This was in bounds, bear hug and carry the guy out of the play. Ball was a little short, but hard to say where Gronk would have ended up had he not been taken out of it. If it's not PI, still at least holding or illegal contact. We don't know which call the flag was for originally, they just said they picked it up. No call to overturn. Tough to believe they decided on 'no penalty' though, out of the obvious options. Doesn't have to be catchable for holding or illegal contact; you can draw those penalties without the ball even being thrown in your direction. Edelman got an illegal contact call earlier in the game from the left side of the field when Brady threw to someone on the right. DB just held him and then knocked him down, still a penalty.