My guess they put too much load on a wire, it melted, shorted and set on fire, then burned through the ceiling causing the collapse. Before you put rigs together learn the load capacity AWG (Americican Wire Gauge) of your power cables.
Unfortunately there are many reasons why that equipment could've caught on fire. You can't just put it down to overloaded cables.
We almost had a fire on our test rig the other day from a shitty power cable. It was only powering ONE 290, and my Kill-a-watt said it was pulling less than 450W at the wall. Yet the cable was so cheap, and it was getting so hot, it was melting and discoloring and charring the testbench it was sitting on. After we unplugged it, it was so brittle you could bend it and snap it in half! As I said, this was pulling less than 450W for less than 24 hours, and the cable was toast.
So yes, it's easier than you think, and only takes one bad cable to ruin everything.