Are you sure? Here's a picture of the situation:
http://img69.imageshack.us/img69/4600/colodc.jpg
Williamson is circled in red. And as you can see, he was already marking a man. But look at the huge space between Colo and Blake (the black arrow). Every player held the line, except Colo which seemingly was more occupied with marking a player that would have been in offside if Colo aligned himself with the rest. With better positioning Colo could have pushed out and blocked Bank's shot. But hey, even Colo got to have a weakness..
Going purely off that still, Colo has been left with no choice but to hang back and try and cover two players, resulting in him marking neither, as it looks like Ferguson has let his man go.