I've learned to just never mount up when running through beastmen areas. Too many casters/ archers. I think I remember the heavy debuff having something like a 20% chance of activating when you take damage while mounted, but it seems like a lot more sometimes. Usually using sprint works better for me in those areas since mobs are quick to attack but don't do any meaningful damage. Could probably make it to any of the beastmen aetherytes walking at the slowest speed and still not worry about dying.