Big rainfall in the desert can get pretty nasty. One reason (among others) is that a lot of deserts rest on caliche, which is a hard layer of clay-ish soil that doesn't absorb water very well. So instead of soaking into the ground, water flows over the ground, and that's how you get flash flooding.

In Black Rock specifically, they're also setting up camp on a literal dried-up lake bed, so when the playa floods it turns back into a huge, muddy lake.