That crack is definitely the result of metal fatigue due to the rack swaying back and forth. You should very carfully inspect the rack attachment point on the other seat stay, because once one side cracks, the sway becomes much more exaggerated, and the other side is stressed to a greater extent. Since you've evidently found a competent welder/brazer, you might have him rienforce the other side just to be safe. I cracked a frame in a similar location -- where the seat stay mounts to the seat cluster. I'm sure it was due to rack sway as well. Lashed it together with parachute chord and road another 250 miles home. By then the other side was cracking and about to fail. I welded and brazed it back together and added a big brass fillet on both seat stays. It's holding up well so far.