Target acquisition and shot placement takes time, it doesn't happen instantaneously. The order of shots just may have been how things played out.I've had to re-watch this several times. Something is really off. This is what I noticed:
1. You can clearly see the USSS spot the shooter.
2. He doesn't initiate and is not taking action...yet.
3. He basically wait for the incoming fire.
4. Finally, he responds when the shots are fired.
Why wait for the shooter to fire first? Shouldn't the USSS sniper take him out first once the threat is in his line of sight?
Maybe the USSS had to wait for the ok, and while they were waiting the failed assassin shot first.