In another life, I ran at least 15 FPs a day, both OC and CC. Today, I work in the petroleum liquids handling industry.
The reason I say FP is non sequitor is that the ambient temperature relative to the FP temp is simply an indication of what happens at the ignition source. In the right concentration, vapors don’t need to be at FP-temp to ignite - if there is an ignition source (LEL).
What I speculate happened here is that vapors from the loading event moved to an ingnition source. The flame followed the vapor trail back to the loading point and “boom”.
Lots of specific things could have been the cause, including improper loading operations. Only extensive forensics will pin-point the cause.
There isn't supposed to be any ignition source in this engineered and controlled environment. If such an ignition source existed, someone must have brought it into the controlled area.