It's a factions bug fixed by UOP in 2008. Previously Rimalus Bruiant was going through mid-life crisis and secretly hated his wife and her dogs.
In my experience Oblivion AI is much better than Skyrim one. And with MOO + OOO it is actually quite good - enemies chase you through loading doors, heal themselves and each other, do not get stuck on scenery, change the tactics when they're losing, aim where you will be, not where you are, etc.
Skyrim AI is much worse - it depends too much on carefully designed arenas for boss fights. Even with mods that suppose to fix Skyrim AI - enemies are still easy to cheese, and your allies are useless. It is not really the case with modded Oblivion - MOO added ability to climb vertical obstacles, OOO added teamwork and going through loading doors - and few high level enemies really can kill you and rape your corpse.