Galling would be very unusual with bronze, which is the reason it is used for turnbuckles. Without a really close examination, this is a mystery. That fact that he said this had also happened a "a few years back" is interesting, suggesting a common factor. Fatigue seems unlikely. The boat is only sailed summers, probably not hard, and I assume the rig was professionally tuned (not slack). This makes fatigue seem unlikely. That the mechanic mentioned the lack lube is interesting. Also though t damamge is not galling, I'm wondering if the last mechanic spun the turnbuckle right up to tension, too fast (there is a LOT of heat generated if the threads are dry) and without lube, doing damamge at that time. Turnbuckles need to be tightened VERY slowly under load. Doing this fast is common, and often unnoticed error, because the damamge is hidden.