Okay, so, I was riding on my horse today with my bow drawn (the Nightingale Bow, to be specific) and I used the favorites hotkey to switch to my Battle Axe (Wuuthrad), only to find that it wasn't unsheathed. I could still fight as if it were in my hand, but Wuuthrad appeared to still be on my back. Removing the weapon with the same hotkey and using it to equip it again fixed this. It only happens when I switch from a bow to a two-handed weapon.