IMHO - Metals can be very strong and rigid (most of the time) because they have specific "networks" of molecular connections that resist change. These network connections can be deformed or degraded after a metal is reheated and joined to another piece of metal. Usually an additional attention to soldering (or welding) the joint makes it as strong or stronger than the metal that's being connected.
However, there is no cheap way to determine how much "change" there has been made to the pieces being heated for connection. In this case an over attention to the strength of the joint - may have actually weakened the tubing nearing the joint. In bike frames with external lugs frame tube the cracks often appear at the edge of the lug. Not really so much unlike what's pictured in this post.
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