Thanks for the replies.

I know what you mean Papewaio, but I think Yuuki is right...I think the route of a high honor unit has no random factors, but can be predicted with certainty based on math (if we only knew the equation). The most common cause of fresh-high-honor troops routing is this: They are surrounded by enemy troops, and most (if not all) friendly troops are routing. This stacks a bunch of morale penalties on them and they crap their pants.

When they fight for a long time it is because there are other friendly units around them, they are not taking a lot of casualties all at once, they are not totally outnumbered by the enemy, and/or they are fighting alongside their taisho-- all of which keeps their morale high.