A lot of people (most notably, Peter King) felt that the Mendenhall fumble was the biggest play in the game. I disagree with this sentiment, but it certainly was the biggest play of the fourth quarter. Now, there's never any excuse for fumbling the ball, but Mendenhall wasn't put in a very good position by the play. It was a weird play that I hadn't seen the Steelers run before, and it certainly made the comeback a lot more difficult.
Figure 1: Mendenhall's fumble
Green Bay came out in 4-4 personnel and lined up in what was basically a 5-3. The three-technique was to the tight end side (the offense's left). The basic scheme was a down/down/kick scheme (everyone blocks down, and you kick out the end man on the line of scrimmage), which is a variation of Power. This play was unique because the center was the puller, and therefore, he was the kick-out man as opposed to the backside guard.
I have two theories on this particular blocking scheme. It was either a change in scheme made at the line of scrimmage because Legursky wasn't sure he could block back on the three-technique to his left, or it was a poorly designed play. The former means that Kemoeatu was supposed to pull, but they kept him back to block the defensive tackle, which meant the center had to pull. Because I think the Steelers are well-coached, I'm going to believe the former.
Anyway, both down blocks went fine, but Legursky got wrong-armed on the kick-out by Ryan Pickett. Getting wrong-armed means that the defender is able to rip through you with his far arm and get inside leverage. This usually bows the blocker back and makes the runner bounce outside. Here, it meant that Legursky got in the way of the fullback and prevented him from getting a block on Clay Matthews. This wall of humanity met Mendenhall, and he lost the ball when Matthews put his helmet right on it.
If the decision to have the center pull was made at the line of scrimmage, I still wouldn't blame Legursky entirely. Certain plays just don't work against certain fronts. Asking your center to snap the ball, pull, and kick out a 300-pound five-technique is just too much. He's never going to get much of a block on this play. I wonder if, after the safety they gave up to the Jets in the regular season, they changed their scheme to allow the center to pull. Against New York, they pulled the guard against a similar look, and Jason Taylor knifed in for the deciding defensive points. Here they tried to pull the center to shore up the backside, and it was just as disastrous. You have to wonder if there was a way to get out of this play once they saw the defensive front.
Also, some great comments, too:
The reason they changed up the blocking scheme on the Mendenhall fumble was because Kemoeatu wasn't in the game. He was on the bench with Trai Essex playing left guard (one of three positions he played during the game). With Essex in there, the Steelers didn't feel comfortable asking him to pull (at least that's my supposition), so they instead had the more mobile Legursky pull. It didn't work out, and Legursky was driven back, but that's the reason as best as I can tell.
Anonymously Social (#7 above) has it right. It was all about preparation.
At 2:08 here, Clay Matthews said that with two weeks to prepare, he'd seen this play on film and the Pack had a plan for it.
At 1:20 and again at 3:30 here, he says he told his defensive lineman what to do--"scrape" or blow up the play inside, and then Matthews intended to come around the fullback (really a TE playing fullback, right?) and in that way, Pickett and Matthews were able to meet at the runner and pop the ball out. Or as it actually played out, Pickett, the center, the tackle, the fullback and Matthews all met at Mendenhall, with Matthews loading up and slamming Mendenhall's ball-carrying arm and ribs with his shoulder.
Of course, (as techvet #9 notes above), the other part of the story is here. Watch the first minute of this video, which starts with GB OLB Coach Greene telling Matthews: "Everybody looks up to Wood(son) for leadership, he's gone. Nobody is f__ing standing up. It is time."
Then Matthews goes on the field, tells Raji and Peprah, "They are pulling this way, I've got a feeling," then shouting to Pickett as the play starts, "Spill it, Pickett, spill it!" On the sidelines, Pickett credits Matthews with a good call on that play, leading to the fumble.