Here's one possibility:
http://news.bbc.co.uk/1/hi/uk/4527864.stm
"A tanker driver who fears he may have sparked the fire at the Buncefield fuel depot has been interviewed by police.
A friend of the driver told the Sun newspaper that fuel ignited when he flicked his engine cut-out switch after detecting a vapour leakage."
that sounds like a very logical explanation. I hope the guy doesn't get in trouble.