If they're actively running away rather than just ignoring you, it's probably because you have a big/powerful army and they don't want to throw down with you. Even lords who hate you will run if you vastly outnumber them.
But if there's a friendly lord who belongs to an enemy faction, he may still attack you despite his high relationship modifier. (I've found that they do sometimes ignore me, but not very often.) They often seem hesitant to attack, at least according to what they say to you before the battle - it's usually something to the effect of "we're buds and all, but our factions are at war so I have to kill you - sorry." But I've had luck with the "do we really have to do this?" conversation option: the couple times I've tried it, the friendly lord will say something like "well, I guess I can let you go, just as a personal favor." But you lose a few points of relationship for it.
