I watched it again.
It was legal.
Watch it again after you read the below.
The center is beat off the snap. He arm bars / hooks the left shoulder of the DT to slow him down. The LG comes off a combo block w/ the LT and has time to drive block the DT due to the hold described above. This is what caused the DT to go to the ground.
It's the same type penalty that gets called on an OT when he's beat on the outside rush and he hooks /arm bars the rusher to slow him.
The described hold above was way worse then the hold on Free earlier in the game.