And using non-explosive fertilizer means no terrorist act?
FP is Non sequitor (and, I think your meaning “auto-ignition temp” which is much higher)
LEL and vapor build-up is more approrpriate.
Ever been inside a empty storage tank on a warm day?
85F outside can get over 110F inside after a few hours of direct sunlight.
I would reckon it was a static electric spark, possibly from prior heavy rain that compressed the roof, and/or the tractor trailer tanker was ungrounded when the transfer connector was inserted.
Matches and sparks tend to be hotter than that.