Any question answered...

I was picturing more of a sudden shearing of half the handlebar at speed rather than a controlled or otherwise anticipated event

It's anticipated, but often rather uncontrolled when it happens. Once a crack is established, the breaking strength is dramatically reduced but the stiffness much less so, so unless you inspect very carefully you don't notice the crack. Then, at the most inopportune moment, what would normally just be a high load cycle (e.g. sprint) becomes a critical overload. The downside of heat treated alloys is that getting all that extra strength is usually accompanied by a much reduced elongation at fracture, so parts don't sag in a potentially recoverable way, they come off in your hand all of a sudden (see Kirichenko) and you end up in the ditch (see GrahamLaming)