After my usual evenings of failure and humiliation in this game I often pop over to Twitch to watch competent players....behave competently. Gives me hope y'know

I'm watching this one large group doing Maw of Lorkhaj on the PS4.
Wow. This crew is really working it. Even the frequent blare of War Horns is coordinated.....in 3...2...1. Yeah, folks are dying. Sure, there is the occasional brain cramp here and there. But they're getting through it.
Then, in the final encounter there are a few party wipes. The raid leader calls for EVERYONE to TAKE OFF THEIR COSTUMES. Huh? Following that - not sure whether it was because of that - they finish up.
Just as the stream is about to go down the streamer asks if we have questions. I try to get my question in about WTF they removed their costumes but it's lost in the noise.
So....what was up with that? What evil do costumes do and, if there is some penalty associated with costumes, why did they wait until the last room to shed them?