Granted, I have little luck with cycling stuff. I poked a hole through a steel seat tube trying to get a post unstuck (my fault, of course). Then there was the 1-year old aluminum frame, in which the BB threads got striped down. (Had to repair it by gluing the BB into the frame!) Heck, I even ended up with a defective Rohloff Speedhub that had to be repaired (covered by their warentee)! But, come on now . . . a freaking TI frame in which the bottom of the seat stay completely cracks open?!?!? (Its an 11 year old custom job, the designer of which just happened to recently retire.) To my (untrained) eye, it doesn't appear to have broken simply along the weld. Guess its a good thing it happened while going slowly uphill, so no danger of crashing. Was able to gingerly pedal the 12 miles home in a low gear. (Haven't heard back from him yet, don't know how this will end up.)

It's a fatigue crack, they go whichever direction is easiest.

I have to say I have never seen a failure there.

There are plenty of examples of broken Ti. Usually there is some kind of a defect in the weld that starts a crack.

First thing I think of is lack of best back purging. This would lead to a contaminated weld. Then as Eric mentioned the crack follows the path of least strength or maybe stiffness. Andy

Spectrum (or Merlin maybe)?

Yes.