There is a lot more wind in the town than most people think. The forecast called for wind, though it was not bad at sunrise on that day.
I'm gonna call STUPID too. I'd like to think that the window cleaners knew the forecast, had experience, and were ready to retreat when the wind picked up. I'd like to think that they had a game plan. OTOH, I've been stunned before.
Maybe they were afraid Steve Wynn would fire them. He's not afraid to drop the ax on anybody. Also of note, it was on private property on a non regulated service.