Cushing tore his ACL as a result of a "peel-back" block, which is now illegal going forward. After the game, Cushing said that it was a "dirty block," and that he hoped the NFL would ban that particular type of block.
The "peel-back" block is perhaps best explained by the NFL's rule book, per Pro Football Talk:
"If a player who is aligned in the tackle box when the ball is snapped moves to a position outside the box, he cannot initiate contact on the side and below the waist against an opponent if: (a) the blocker is moving toward his own end line; and (b) he approached the opponent from behind or from the side."
The rule contains the following note: "If the near shoulder of the blocker contacts the front of his opponent's body, the ‘peel back' block is legal."
Essentially, it is now illegal to block an opponent from the blindside, at his waist or below, from anywhere on the field. At the time, Matt Slauson's block on Cushing wasn't illegal, but now it is -- and it's quite easy to see why the block is illegal. Unfortunately, it took a torn ACL to Cushing to really illustrate the dangers of the "peel-back" block.
Here's a look at the injury to Cushing, suffered in Week 5 against the Jets.
Cushing had no way to defend himself against the block, and though legal by the letter of the law at the time, it was undoubtedly a dirty play by Slauson, who went after Cushing's lower half.
Such a block will now draw a 15-yard penalty.