There's another thread going on speed wobble, I'll try to post in a sec, and someone mentioned a podcast on the subject, and darned if they didn't mention a Ti bike on a very fast downhill, got into serious speed wobble, was barely able to stop and sit down to calm down, and darned if there wasn't a crack exactly like you posted, across the top of the downtube spanning the shifter bosses. I think the severe speed wobble caused the crack, not vice versa, but not certain.
EDIT: Post #35, the Silca podcast, really long, like an hour, the Ti bike part is in there somewhere:
Resonance danger