Since SpaceX has isolated the AMOS-6 Falcon 9 pad explosion to the upper stage LOX tank during fueling, it seems useful to consider in general terms what was happening at the time and what materials and equipment were around. Rocket explosions usually involve firing engines and/or structural failures in flight, and I have little sense how this explosion was possible.
The current SpaceX statement is thus:
At approximately 9:07 am ET, during a standard pre-launch static fire test for the AMOS-6 mission, there was an anomaly at SpaceX's Cape Canaveral Space Launch Complex 40 resulting in loss of the vehicle.
The anomaly originated around the upper stage oxygen tank and occurred during propellant loading of the vehicle. Per standard operating procedure, all personnel were clear of the pad there there were no injuries.
I have read that LOX makes most anything extremely explosive and it takes very little to trigger that. How little does it take?
Would a spark or some other trigger have been necessary, and if so, what sources of such a thing were nearby? Would it have been easy for some source of ignition to have ended up there from the environment?
Would a leak of the equipment have been necessary for this to occur?