There have been several on-stand fires:
Egyptair, Cairo:
Accident: Egyptair B772 at Cairo on Jul 29th 2011, cockpit fire
Corendon, Antalya:
Accident: Corendon B738 at Antalya on Oct 14th 2012, cockpit fire
This fire is too far back to be a cockpit fire. But a common link with the other two, is that these airlines use(d) eau de cologne for ritual cleansing, prior to eating. Thus the catering included eau de cologne bottles, which tend to get stored on the oxy mask, as this is a convenient shelf.
The reason that these fires happen at the gate, is that the crew give the oxy a good squirt in the preflight, and if the cologne bottle has a bit of a leak (leaking into the oxy mask box) you have all the ingredients for an instant fire.
Probably not true for this fire, but certainly a good probability for the other two.