So what's going on? 

A rare, but not unprecedented (it actually happened last year as well, as you can see here) weather phenomenon that the canyons see from time-to-time, called a total cloud inversion. PBS NewsHour explains what this is — and why the phenomenon won't last long:

The clouds materialize when cold air gets caught between the Earth's surface and warmer air above, according to the National Weather Service. Weather forecasters expect these clouds to dissipate gradually as a cold weather systemmoves in, bringing the first snow of the season to the national landmark.

In other words, see it while you can, winter is coming. For those of us not in driving distance, here's what it looked like to watch the clouds slowly spill up over to the top of the canyon — a process that took only 15 minutes: