Thank you, bambalam and Watson, that solves this mystery.
I thought it was common cause that plastic containers were unsuitable for fuel, because of risk of static electricity, and that the plastic container ruptures when the pressure inside increases (provided the lid seals in the first place). Could there be a similar problem with standard steel jerry cans? They seem to be 100% airtight, can be properly bonded with an earth cable and seem to withstand high pressure differentials without deforming a bit.
The only thing that’s puzzling me now is why was the pilot's seat belt not fastened...