should be obvious
I don't have any inside knowledge but would think it would be obvious, it's the best flight for the airline to cancel when they need a spare aircraft.
This might well be because the schedule is awkward so taking this flight out opens many opens for shuffling flights and crews when they need a spare plane to replace a broken one. It could be because there is a decent alternative in a few hours/to a nearby airport to transfer the displaced passengers to.
And most likely, loads are light so it would annoy the least number of people. At the end of the day it's a purely commercial decision. If it's annoying you and you have a valuable spend with BMI, write them a letter pointing out your valuable spend is at risk of going to another airline!
G