Firefighters combat large propane-yard fire causing property damage in Phoenix

I was wondering what was burning last night. So much smoke it didn't look like a small structure fire.

This video really shows it. A couple of things I picked up:

The fire crews were cycling out in 15 minute rotations to a cooldown space (I don't think I even stay conscious for 15 mins in all that gear!)
One theory is that pressure relief valve tripped because of extreme heat and started venting propane out (by design), but then some unknown ignition source ignited that (which I presume should not happen).

Firefighters combat large propane-yard fire causing property damage in Phoenix | Spyke