I remember a friend of mine who used to ride horses and one day her and her sister fell off the horse on top of each other. It would have been hilarious to watch had i been there, but the girl on the button fell down and was fine. When her sister landed on top of her she broke her leg and had a small spinal fracture, all thanks to sister falling on top of her.
Leading me to believe that is it all about how you fall. Naturally I horseman would understand how to properly fall off of his trusted steed. Whether he takes damage or not should all fall under the calculations.
Speed chance for damage
standing still to low - no damage
medium to gallop - low to medium damage
Terrain
Flat - little to no damage
Down hill - less damage but sliding
up hill - face first medium to high damage
What you hit.
grass - softer little to no damage
rocks - little damage to high damage
trees - high blunt damage
Then you apply the formula
Galloping down hill and your horse dies and you hit a rock - instant kill.
Trotting along on a flat plain and horse has a heart attack and dies and you land on the soft grass - Relaxing
In half gallop going up hill and your horse dies sending you flying into a branch of a tree and then hit the grass. - branch takes half of your health and the fall from the branch to the ground only does 5% of your health.
That could work but at the same time who cares they are off their horse already the animation to get up lets you hit them without retaliation perfect to finish the job you started.