Potterburn is a decent estate of strength 7, located on the western edge of our campaign map - and caught between Lords Monkton and Whitcaster, the two Perkins brothers on opposite sides of the war. With the lords having picked off the low-hanging fruit, they are now growing in strength to the stage where Potterburn can no longer stay neutral.
First to move is Lord Monkton, our solid-but-unimaginative Yorkist. He marches west from Fishdale and arrives in Potterburn, with a force roughly equal to the local host. Potterburn is also pro-Yorkist, but has no intention of simply rolling over to a bigger neighbour. He holes up in the manor-house for a siege - or, more accurately, a prolonged stand-off while the two loyalists negotiate with each other about influence, titles, and other baubles.
So far, so mundane - except that Lord Whitcaster then marches south into Potterburn with his Lancastrian rebel army! This move is largely forced on the Lord Whitcaster, as every other direction has him hemmed in by more powerful foes. The active Lancastrian decides to try his luck against Potterburn and his brother, since after all - capturing or killing his brother on the battlefield will certainly boost his chances.
We now have the bizarre situation of three equally-sized armies all contesting the same area. It is likely that the brothers Monkton and Whitcaster will fight each other, so Potterburn can theoretically sit back behind his walls and let them scrap away; the survivor will almost certainly be too weak to pursue a siege. However, there is also the chance that by entering a fight he would tip the battle one way or the other and could basically make all the difference between victory or defeat.
There's also the matter of Lord Potterburn's characteristics: Depraved morals, an enthusiastic fighter, and astoundingly treacherous! The man's as predictable as a lunatic. Basically because he is a lunatic, but just one that happens to have a title, and a large army at his disposal.
[Based on reading about 'solo campaigns', the advice I got for a complex situation was to come up with several plausibly coherent courses of action per participant, and then roll a dice to randomly select a way for them to jump. I'll do this for our battle, which may be a straight York vs Lancaster affair, or a far more random one with an unpredictable third-party taking to the field!]