I was killing a quillboar Geomancer in the Barrens while a warrior was several mobs down killing a defender, there was 2 quillboar hunters between us, my quillboar dropped aggro from me and charged at the warrior, aggroing 1 of the hunters.
Not sure what could have caused this as he was level 13 and meleeing unable to influence the aggro table (he was between 30-40 yards away), and especially on hardcore that was panic inducing.