I've attended a few C&CC rallies on the deer park at Eastnor Castle although nothing on the scale of a National Rally.
I doubt if there would have been much of a problem with the haulage of generators, toilets, marquees etc because there are tarmac maintenance roads running through and around various parts of the site, which often hosts rock concerts, firework festivals and other fairly major events.
There are some reasonably level and gently sloping pitching areas on the deer park but most of these are surrounded by hilly terrain forming a sort of giant bowl, so my guess is that some of the fishing pools have burst their banks and waterlogged the flatter areas which would be made worse by water running down from the hills.
It might dry out but if it remains boggy the CC could have a lot of disgruntled members on it's hands, so one can hardly blame them for taking care of their members best interests.