Burning man is in the Black Rock Desert. The Black Rock Desert is an endorheic basin. There are no outflows. The lake bed itself is 1,000 sq miles. The drainage that drains into the lake bed is 11,600 sq miles.
An inch of rain isn't a big deal, but when you concentrate it 11:1 it becomes very serious very quickly.
Desert ground is hard and mostly impervious. That 1" of rain will mostly run off from the surrounding area into the now not so dry lakebed. There's a dry lakebed not far from me a little smaller than up there and an inch of rain can easily lead to a foot of standing water if the rain happens over a widespread area in the watershed of that lakebed. I hope they packed stilts!
An inch of rain isn't a big deal, but when you concentrate it 11:1 it becomes very serious very quickly.