I've been looking around for any mention of this bug but haven't come across a thread.

While on you're horse you ride past a hostile, the music changes and they attack you. This is still the same with humans, elves and the other races. However, when you ride past creatures the music stay the same and if you jump off you're horse to engage you will not be attack until you're horse is dead. If you don't have a horse with you however the creature will attack you as normal.

The bug is triggered by the knights of the nine plug-in ones you start the quest "the sword of the crusader". When you have collected the crusader's armor and the mase you go back to the "Priory of the nine" Lathon will confront you and give you the Greaves, after this the bug takes effect.

Im looking for any info on this bug.

I believe that one of the relics makes animals neutral to you (but not to your horse). Check your buffs and see if it still happens when the buff is not active.