I may be wrong - I was once - 23 Oct 1971 - wedding day - but, without seeing it in the flesh, I still don't think the holes are to blame. It's probably started where the internal thread crosses that deep score mark on the outside, and then spread along the score mark. If the crank is way short of the end of the nut, as Ogri suggests may be the case, the break may have been started when it was tightened, and the crack could have been growing until there just isn't enough metal for the job, then you'd have a lot of noise and no drive.
Ogri, I've seen wiring holes drilled like that, not very successful, the corner where the two holes meet tends to cut into the wire. Probably someone who was P***d off breaking drills trying to drill accross the corners - I used to buy 1/16" drills 10 at a time, until I finally sussed it.
H