That is very odd as its one of their major exhibits. Can only think its because they are trying to integrate the small seperate group with the main one. Can't think of any other reason. They don't go outside that much though my last visit I did actually see some of them, about four, using the outdoors.
I have had a reply from Twycross regarding the closure: it is closed due to fighting within the group. They said that 'due to some injuries sustained' the chimps are being given space so they can be cared for by the keepers and vet team.
No mention of any names, but did imply it was a challenge to the dominant male.