Re: Could use an explanation as to how I died...

Looks like a classic case of latency + hitbox.

You character model just like every other character models are surrounded by invisible blocks slightly outlining your character model. 
Within these boxes your model is build and rendered, Hitting inside of these boxes despite it being a hit in the model visual's will count as a hit. 
And I'm almost certain this is the case in your video. 

I made a quick sketch in microsoft paint to give an idea what a hitbox is. 
The green lines represent the hitbox and within them the model is rendered. 
It could be that a tiny part of your hitbox was still in their line of sight, besides that from your angle it might appear you're fully covered and behind the wall. 
But from their perspective a shoulder or a leg could still be sticking out and since you were on low health 1 shot would have killed you.