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.
on top of that the AWG is usually on Electrition code to install enough for that circut braker. so If a circut braker is 15 amp they will run a 12-14 guage wire.