[Oldie Goldie]

I have a theory. One of the three big balls bearings came out while the other two wouldn't slide out.

I didn't think it was important. I concentrated on cleaning all the other stuff.
But if that black gunk has gummed up around the inside of the two holes, than the two balls aren't coming out all the way to mate with the narrow in the inside shaft.

So in theory only one ball is holding the shaft on (the other three only bearly) and any roughness in the surfaces would cause the thing to pop out the first few turns.