I suspect there is a combination of things.
Firstly why would defra close them down if all oldbirds/stock are vaccinated and youngbirds onsite are healty?
The possible cause could be stress, the birds are weaned, jabbed, shoved in boxes, sent of for delivery which in many cases is now taking far longer than it has done in previous years then being put into a strange loft etc.
A stressed bird will be more prone to going downhill and this may then allow a desease to get a hold.
Always, Always keep any new birds seperate!