Any heavy rust on the discs tends to rip the rear pads to shreds, it can be especially bad if the van has been left with the handbrake on, as where the pads have been in contact with the disc there will be a clean rust free patch with a ridge on the face of the disc where the clean bit meets the rusty bit, its that ridge that chews up the pads.
The less you use the van the more chance there is of heavy rust forming, if you use it frequently you get just a light surface film forming which is easily removed by the pads after only a few miles.