May I be bold enough to go one step futher and hypothosise thus:
As I have previously posted, there were a number of UK regional airports trying to establish the route to BHD, they were all offering considerable concessions to whichever airline took up the route and all of them had greater revenue potential than the HUY - BHD service.
I happen to know that, believe it or not, Belfast City Airport have a local authority planning permission limit regarding the number of passengers allowed to use the terminal each year (2 million i think) and there is also a strict limit on the opening times of the airfield.
For last couple of years, BHD have been sailing very close to the wind regarding the passenger figures, if the local authority can prove that there aren't the routes to sustain the agreed limit, they can reduce the numbers permitted.
This leads me to suspect that BHD may have been agreeing to certain routes in advance to convince the planners that there is an established need by agreeing to additional services (such as BHD - HUY), which can then be thrown into the bin when the summer season comes along and more viable and sustainable (bums to the sun) flights come along.
Pure theory of course, but ...................