jimmuller 

That's what I've been thinking about too. The chainstays and indeed the entire BB are in fact under tension by the rider's weight acting on the rear triangle.

However the chain would be pulling the rear axle forward, and since the chain is on right side that side's tension would see a lower maximum than the left side, and probably lower average tension too. Hence a tension separation is more likely on the left.

No matter how hard the rider cranks on the pedals or pulls upward on the handlebar, the rider's weight on the BB as far as the rear triangle sees it will never exceed his/her downward dynamic load. If the rider is more or less stationary vertically that load will be just the rider's weight. However a rider could bounce up and down dynamically to generate more force on the pedals. The bigger the rider the more dynamic load is possible. (Little 160lb guys like me can't do much in that regard.)

Twisting forces on the BB might also be a factor. One would think the ST and DT would take most of that sort of stress, but perhaps it can alternately compress one chainstay and tension the other.

So one must conclude that TBB pedals with vigor.

Mr. Bullet, a question. Do you recall where in your pedal stoke this failure occurred? That might be a clue.