There's nothing scarier for a quarterback than an unblocked pass rusher.
If you were a quarterback for the Broncos, Redskins or Cardinals in 2010, you had plenty of reasons to be frightened.
Denver, Washington and Arizona were the only NFL teams to reach double digits in unblocked sacks.
There are a couple of ways that a blitzer can pick up the easiest of easy sacks. The simplest explanation is that confusion along the offensive line (or among running backs assigned for blitz pickup) lets someone come free even though the blocking scheme should have accounted for him. On other plays, especially if the defense calls a corner blitz, the unblocked blitzer is the responsibility of the quarterback and wide receivers -- the quarterback has to recognize it and the receiver has to break off his route so that the QB can get rid of the ball before the blitzer arrives.
A high number of sacks where the rusher comes free would appear to indicate either communication issues, play design problems or a quarterback who doesn't always recognize the corner blitz. With the Broncos, the biggest problem was simply assignment confusion. On five of the 10 sacks, a rusher came free because an offensive lineman or back appeared to fail to carry out their proper assignment.
For Arizona the problems, as you may expect, came largely from inexperience. Four times a corner came off the edge unblocked with the Cardinals' revolving cast of quarterbacks struggling to recognize the blitz. Brandon Keith was also part of the problem -- two of the sacks came when he appeared to miss an assignment.
The Redskins didn't have any one problem that led to the 10 sacks. There were backs who couldn't get across the formation quick enough, screen plays that were blown up, designed rollouts that didn't manage to fool a defensive end and DB blitzes off the edge.