The University had a long standing arrangement on the day which finished at 15:00, thus meaning the car parks were unavailable to the club for match day parking until the University event finished.
And in answer to the original question, it was moved due to the bonfire celebrations on the 5th in Lewes due to the large numbers who would be heading to Lewes for that around the same time as the fans would have been leaving the ground and also the impact of street closures etc in Lewes on our fans who park up in Lewes.