Kerrev "Insta"-kill

I did a quick Sundered Doctrine with some randoms and some guy asked for a div on final boss. One guy used div and when boss DPS started the guy kill him in literally 5 seconds. The damage results came back, and we only did around 300k each. What happened? Is there a glitch or something? He kicked me right after we got gear so I wasn't able to inspect or look at their gear. Does anyone know what that could be?

Edit: After reading some of the comments it appears I was in a dungeon run with a cheater. I've never heard of the term "netlimit" before but that seems like it would make the most sense. Thank you everyone and my apologies, I 100% would have reported them had I known of this. I can review my dungeon report to get their username, but I am not sure what good that will do at this point. Advice is appreciated.