The first time I really noticed it was at the BYU/Oklahoma game in Arlington, TX in 2009 where I saw not 1, but 2 Yewt fans dressed to the hilt in their gear. It was pretty cool because this was at the end of the game, they were both leaving in the same area within the vicinity of the BYU section at Jerry World, and both were being slightly mocked by all the leaving BYU fans for their attire right after the game (which we won, of course). They both had heads down and you could tell they wished they were somewhere else. Probably thought we were going to get killed, came with Y fan families, and couldn't believe we won. It was awesome.
Since then, I've probably been to a dozen events in both Las Vegas and Phoenix — football and basketball games and a couple LV bowls (not including the 2015 game) — where I've seen it almost every time I've gone to a non-Yewt BYU event. There will be at least a couple of crimson-attired Yewts in full gear.
I can't explain it, it's very odd. I think they think it gets in our heads for some reason, but they just look like idiots.
BTW, BYU has won almost every one of these events where I've witnessed this.
GO COUGS.