Don't know why I am bothering to respond, but here goes. You didn't use followthrough after ambush, you used an instant snipe(watch the snap shot buff disappear when you use it). Your followthrough does not go on cooldown and you do not use it once during the vid. I am sure it does hit for more than 2k, which makes me wonder why you used snipe in the first place. Also, your target is being attacked by a marauder at the same time, which explains the dramatic loss of HP combined with the fact that the target is a sage who is probably a fresh 50 with terrible gear with somewhere around 11k HP.
ps nice entrench in a terrible spot with no one attacking you