To be honest, no, I never experienced that problem. I would be hazarding a wild guess here, but does your rear shock ever bottom out heavily? The rear shock weld joints (and subsequent) in that section may have made the metal brittle. Alternatively if the bushes in the swingarm wore unevenly at any point this creates a twist in the swing arm action that will come up through the bars below. The whole bikes frame and rigidity centres arount that section and so is subject to fatigue.
BTW if at all viable use silver solder rather than welding it to plug any weeping to avoid causing the joint becoming more brittle.