Former Premier League referee Graham Poll believes Webb was looking to make a "big call" after being appointed for the FA Cup final between Chelsea and Everton next month.
"Howard Webb found out just this week that he'd got the cup final and I think what you do is you think, 'I've got to prove I'm the best referee' and you look for a big call," Poll said on Setanta's Football Matters.
"It's the wrong thing to do and I think Howard's looked for a big call - nothing to do with favouring Manchester United, nothing to do with the situation in the game.
"The interesting thing for me is that Howard Webb, when he knows he's got something right, is very strong in his body language - very strong indeed.
"After the decision, he was walking away, getting away from that area. The players, he knows, are incensed.
"As a referee, you have that feeling that you got it wrong in that situation."
Reading 5/4, Birmingham 7/4, Draw 12/5