The first I heard about one going was about 5 years ago.
There have been a few since(or even before) as mentioned above but nobody has been hurt as of yet.

From what I know it has always been the right barrel that blew up and always near where the brazing has been done as in chamber area.

-My own personal view is that it has to do with the metal compostion of the steel used(those barrels were never designed to fire high pressure modern powder)

-Temper embrittlement to a degree (some explanation here here
)Not quite the same as what is explained in that doc but heading towards that direction. One doesn't harden a barrel...
Don't forget in those days one didn't have a blowtorch nor electrical heating elements to heat up the chopper lumps to brazing temperature which is very high as one wants to brazing to flow everywhere, more lickely heated in forge in some way, I have never asked anyone how it used to be done.
The temper embrittlement could happen by the barrel cooling down to quickly and actually hardening it in places, an open window and a draft in the workshop could do that one as far as I know.

-Heating high carbon steel up to great temperature also has different effects as in weakening it, you might have to ask 2piper and ChuckH on gunshop they corrected me in a way last time I came out with something similar..

The reason I think like that is as there was a piece broken off..
This is very good read about an analysis after a gun blew up
http://www.thegunzone.com/m1akb/762r.html (found through a link on accuratreloading..)
He doesn't mention where the heat of the metal was for to long.

-Metal fatigue, continual exposure to high pressure, from what I know it was always the right barrel..right barrel that's most lickely to be most shot and used.
Don't see any relevance with looking for barrel obstructions or reloading error or ammo error.

Those are just my personal views, I'm not a metallurgist nor claim to be one but like you interested in what causes them.
These things unfortunately do happen.

.50AccuracyInternational with error in barrel steel?

Hope it makes some sense and is helpfull.
Let us know if you find out some more, I can see an accident happening with someone on safari.

Arnold