the WWI battlefields still hold a nightmare of unexploded shells and mines. The mines consist of tens of thousands of lbs. of explosive buried in tunnels underground, some took years to dig.

https://www.chemistryworld.com/features/the-great-war-clean-up/3009456.article]

https://news.nationalgeographic.com/2017/06/biggest-blast-before-atomic-bombs-messines-world-war/]

At Messines 19 planted mines went off, some of it didn't its tonnage of explosives remains underground .

I was there last yr, there is a farm directly above one of these sites today. someday....someday.

https://en.wikipedia.org/wiki/Mines_in_the_Battle_of_Messines_(1917)]

There's unexploded ordinance in battle fields all over the world. I have heard that they're parts of France that are off limits because of WW I ordinance. I remember an unexploded V2 rocket was found in an English harbor a few years ago. The explosives become more unstable the older they are. Lucky the farmer did not trigger that thing when he was working the field.

In the middle of the night, in the middle of a field, this does seem to have been 'spontaneous'.

Many used delayed fuses or were intended to become mines that would be set off by slight tilting or magnets used by those trying to disable it. The ground could have shifted enough to set it off or the detenator became so unstable from age that a breeze blowing across the field would do.

Here's a link to an interesting wiki article on unexploded ordinance.
https://en.wikipedia.org/wiki/Unexploded_ordnance