Here's the news:
This explosion was probably due to our prevalent use of hydrocarbon-based refrigerants used in newer equipment nowadays.
HC refrigerants have come into use for having a VERY low score on the GWP (global warning potential) index. Otherwise, refrigerants like R134A and R404A score very high.
That's all relevant to those concerned about greenhouse gases and the man-made impact on climate change.
In the case of this Whirlpool refrigerator, it' probably used R600A (Isobutane).
SO...why did it explode?
I don't know, but here's my take on it:
R600A is heavier than air, so a leak of it could have pooled into the bottom of the freezer/fridge compartment as a vapor (maybe the same level as the defrost heater?) or stirred into the freezer section's air by the evap fan. I tend to suspect it was during the defrost cycle when the fans were OFF.
Somewhere in there was the transition level between the air and fuel vapor which could have had the right flammable mix of 1.8 to 9.6% fuel-to-air. At THAT point, all that's needed was a source of ignition.
Perhaps a not-so-good electrical connection to the evap fan, the defrost heater...or the DTFD (defrost termination-fan delay). Electricity is applied to that component, a spark occurred and...
Some folks on another forum (where I found this news) commented about nothing being scorched or burned.
If it had exploded from the proper fuel/air mix with an ignition source, there'd be some signs of fire...RIGHT?
Not necessarily. An explosion from gases in vapor form RARELY results in a fire. It quickly blows itself out as all of the flammable gas is almost immediately consumed in the explosion.
I've witnessed that myself (very recently)...and from reading about the New London Texas school explosion in 1937 stating that, although the building was completely destroyed and MANY lives lost, there was no evidence of any fire in the aftermath.
Beings this was a household fridge, the amount of HC refrigerant couldn't have been more than 2.0 oz (57 grams) per design limitations. That may not seem like much, but that'd be like dumping gas from twelve Bic lighters into a small, airtight space and igniting it. When it lights off, it's GOING to expand VERY VIOLENTLY.
Beings that it was enclosed in such a tight structure, that made it all the more violent since that structure (the refrigerator) rapidly expanded outward.