It's things they don't understand how to handle. The typical meta player doesn't comprehend what isn't used on a daily basis against him or her. This results in the player feeling massively insignificant while facing such players, as they litteraly do no not have a counter that they can effectively deploy against you. They also start to get angrier and angrier as you kill them, so they get more aggressive and fail (Tilt, a term stolen from poker and used in gaming now) and the cycle continues. Luckily enough for me personally, I used to played 3v3s (before the sweat scene was really popular with (and against when we were bored) against my friend who mained a pulse rifle and Fusions. Specifically, he had a Plug.One and a Purifier, that he felt comfortable using at ranges that would make people break TV screens. One specific instance, he was on top of A stairs in Burning Shrine and straight up one shot a kid walking through the archway to C from full health. Another instance, he mapped a kid on Cauldron from top heavy to one of the opening doors. I say it's lucky I played him because he taught me how to play against fusions specifically. One wrong step and I would be blown away well outside fusion range. And because he was a cautious, slower player and I was aggressive I figured out damn fast what to do against him.