What a frightening event. Lucky for you the safety glass was there.
I doubt that anyone could tell you for sure what caused it. Maybe a manufacturing defect, maybe the neck was rapped sometime in the past, creating a little stress fracture that finally let loose. Maybe a combination of little things that finally added up to one big thing.
It's a little surprising that there aren't more such incidents, considering how some of these old TVs get jumbled around in garages & basements & attics over the decades, exposed to hard knocks and temperature extremes.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html