So I play Jett but it happens so often that I just peek an enemy get a headshot and instantly dash behind cover really fast, but still die even tho i was already out of the action. Is this because of Lag or something i dont understand

Just got something call a "simultaneous finish - spike detonated." I looked this up and couldn't find anything on it, does anyone know what it means and what triggers this? It was under the victory banner.