He was ambushed — no chance to draw-down and fire.
I find that unlikely because when you are working alone in a high intensity drug trafficking area you are always on high alert for just that kind of thing.
My best guess is that he was in the process of arresting a large group which put him in close proximity to the aliens/smugglers. At that point one or more jumped him, got him on the ground and killed him.
Its every agents worst fear. Agents frequently apprehend groups of 10 to 20 aliens at a time by themselves. Even if only five or six put the bum rush on you you are in big trouble because as you focus on the first attacker the others can quickly move in.
That Nat-GO series 'Border Wars' give you the false impression that the agents work in groups of 5 to 10 and there is always a helo just moments away.
Its nothing like that in places like Big Ben where your backup may be hours away and if you have a partner he is likely to be a 100 yards or so away covering the other trail they could be on. A helo, heck you could go days or weeks without seeing one.
My total speculation is that this was not drug related. The cartels know that killing US agents is very bad for business risks the US deploying the military along the border.
My guess is that the agent encountered a group of crossers and wile in the process of securing them he was jumped by one or more who were prior deported convicted felons who knew that a trip to the processing center meant a strait path to federal prison.
This is a very dangerous profession and the agents go out every day and take this same chance. They need our support.