This bug has been there since mounts were added, what was said about armor stats is likely most of the problem, but since I always ran glass I'm used to having the base 11k hp, the mount has 2k-3k, when your first dismounted, mine typically changes to 5k then 11k in a very short window, as the entire UI disappears sometimes you'll see it, getting dismounted cc's you, all they need to do is a short sustained burst, rapid fire is 99% the culprit because it can't be a single hard hit, and the few skills capable are all melee range.