Wow, that's really too bad! Climate change might have done it, or changing strings. It's just a guess, but I think that if it's NOS, it had those strings on it since '88 and no change in tension (playing, bending, whammy bar antics, etc.), so everything settled in at just that tension. When you popped the strings off and then put new ones on ... that might have been the first "flex" the neck had seen since it was in the factory. Maybe that did it.
Mysteriously, the '88 777DY I bought a month ago has no stress crack ... could it be the only one in history to survive unscathed? Ha ha.