Titanium (alloy, as used for bike frames) is about a third less dense than steel, but also less stiff than steel in about the same ratio.
Therefore, large diameter tubes make a lot of sense with Ti, and Ti frames can be too flexy if made "down to a weight" and certainly if made with the sort of size tubes used in steel......and Ti frames aren't usually one third lighter than steel!
Welding Ti is a touchy business, as somebody already said, but also it should be kept hot for a while to de-stress after welding.....there is a proper word....is it annealing? Some builders seem to be more keen to do this properly than others.
There are some Brucey words of wisdom here...viewtopic.php?p=1267788#p1267788.....including the frightening thought that Ti tapered stays might be welded up from sheet, rather than extruded like steel. (Do I mean extruded? having trouble with words !)
Interesting that the crack doesn't appear to involve the cable exit hole.....that hole wasn't a stress-raiser!