My first thought was that somebody probably was trying to sneak a smoke. Many people are in the condition of having to use oxygen tanks because of years of smoking, and it is extremely difficult to stop. My aunt with emphysema and many other health problems must carry around an oxygen tank AND she also smokes at least a pack a day. Of course she turns off her tank when she lights up, but if you're in a bus with many others on tanks you can't be sure they are all turned off.
I'm not blaming anybody, it's just a likely scenario for what happened.
I wonder if the authorities checked all evacuees for lighters and cigs.