Could be either fallout with the FA or other referees, I've heard Jeff Winter and Mark Halsey talk about how refs are like behind closed doors with each other, so could be him (Clattenburg) crying about something!
I have noticed however in his 29 games refereed this season in all comps, he's dished out 101 yellow cards and 4 reds! I wouldn't be surprised if the FA have pulled him up on this and told him that his disciplinary record is not good enough for someone regarded as our best referee!
He reefed the derby at the start of the season and booked 6 players (4 United and 2 City), where as last night Martin Atkinson booked 2 (1 City, 1 United) and sent off 1 (United) allowing much of the game to flow and was very lenient on blowing up for fouls! Much like Mike Dean did in the FA Cup Semi-Final 2011