Perhaps they had already worked out what they would write up

In all the coverage of the "plodgate" incident involving Andrew Mitchell, no-one seems to have an answer to the question: why did the police refuse to open the gates for Andrew Mitchell on that particular evening when they had opened the gates on all previous occasions?

Can someone please tell me the answer to that question.

Were the police trying to provoke Andrew Mitchell?  Perhaps they had already worked out what they would write up in their police log.  Perhaps they even had arranged an impartial "onlooker" to witness the exchange.