I've been reading this and decided to dig out my copy of "Northern and it's subsidiaries - from No Place to Success", and see if this sheds any light on the vehicle swaps.
Two entries stand out for 1986:-
"Northern found it necessary to hire 2 Bova Europa coaches from United for use on additional National Express Rapide services introduced with the summer timetable on 27 April and in exchange for these, 2 of Northern National Omnibus Co's Leyland Tigers were loaned to United (who gave them United fleet names and numbers)."
Later, it is stated that 2 Plaxton-bodied Leyland Royal Tiger coaches to Rapide specification were hired from Yorkshire Traction in December 1986 which allowed the Bovas to go back to United and the Tigers to come back to Northern. Apparently one Tiger had gained Northumbria fleet names by then!

Rather a long post but hope that puts some meat on the bones. It seems to prove that the swaps are linked.