Firstly how are the guys doing that were injured?
Bre901 is correct, pure oxygen will react with most things that have a flash point. Even metals at high pressure or temperatures. The more available Oxygen the lower the flash point becomes. Once low enough, any source of ignition will start a fire, more oxygen the faster the fire will burn. That is why cotton clothing should be worn, Synthetics easily produce static sparks.
The fire storms in USA are a typical example. If you think Bread burns in Toaster, dip it in Liquid Oxygen, light it and it will burn like an Signal flare!
Most oxygen systems are made free of anything combustable. (within limits)Clynders are best made from Stainless steel or polished mild steel. Dry Oxygen is used to prevent any rust forming; FeO2 to FeO3, which becomes pourus and can flake off. If a high pressure clynder became pourus the passing Oxygen can accelerate the reaction (which also produces some heat). It also severely diminishes the structual rigidity of the clynder. Thats why they have a shelf life and should be discarded if completely depressurised. If conditions are right the bottle can explode due to the structual faliure, sparks from flying metal can start a fire/ explosion and can destroy an Aircraft. Hence NO PASSENGERS on board whilst charging.
Slow charging is used to minimise leaks, but also reduces vibration, structual stresses, prevents any icing or heat build up within the pipe work and doesn't freeze or melt open the valves.
Hope this explains the procedures we follow when charging Oxygen.