[Torin]

Attack rearmost fails if theres units in the path or near the path so they "lure" or "taunt" your units to melee.
Theres a check. I dont know what checks but your units are not lured by commanders unless they are too close. But instead looks for "soldiers" or non commanders.
Perhaps checks morale or something in the enemys stats.

You can force the ai to go to one side placing your frontmost group un that side and flankers on the opposite. But human opponents can prevent that putting their own flankers on both sides so you cannot slip trough. Slow flankers are best to stop enemy cavalry so they dont venture too far ahead.