The tyre pressure warning light came on just before it went bang.
Thats what lead me to suspect rapid deflation, which could've been caused by a nail or something. If the sidewalls had just spontaneously failed it would've just gone bang and the TPMS wouldn't have had a chance to sense the difference in rolling radius beforehand.
Either way I'm glad nobody was hurt and that it has been resolved, poor CS notwithstanding.