Could it be that a number of the Doncaster passengers origionated in the Leeds area? I know its a long shot but perhaps they are trying to get a foot in the door.
Highly unlikely, though if this is the case then it's more likely to attract those in the Durham Tees Valley catchment area as this is where the highest amount of PAX seem to come from on the EZY flights at DSA, despite LBA only currently having GVA and NCL probably closer to most.