Why do monsters attack certain targets?

[Wyrm]

E) I remember a long time ago, skeletons (maybe other minions, I don't remember) would attack your BW/BP if they weren't already attacking an enemy, perhaps this is some lost remnant of that which wasn't removed when that was? The explanation for them attacking was that even though they were soulless and undead, they still didn't like you using their friends as walls (or something to that effect). Of course, BW and BP have had a long history of oddities, including their dropping items when destroyed.