If I had to guess at the play that did it, here's one that would be a good place to look first. We lined up Cannon, Fleming, Gronk and Bennett to the right, and Cannon and Fleming doubled Watt. Hit him hard right off the LOS, and he went down right away in a kinda awkward position.
From that point onward he really seemed to be struggling, in a way I haven't seen him struggle before. Both Cannon and Thuney handled him with ease; much easier than they handled lesser players that they blocked on other plays. For example, on the very next play Watt tried to bull rush Cannon, but Cannon handles him very easily - he's too high up, has no leverage, and doesn't look like he's generating much power as a result:
And just a few plays after that, he got dropped on his ass again, this time on a Cannon / Gronk double team:
It's unusual to see Watt get knocked on his back once in a game, let alone twice in a couple plays like that, even against double teams. And maybe I'm reading too much into it, but it sure looks like he was too high and stiff on those last couple plays, which could well be a sign that his back was hurting him.