A fatigue crack can move through material very slowly, even over a period of years. The final failure through the overloaded remaining material can happen at any load.
I've seen fatigue cracks that leave only 5% of the original material remaining before a rapid final break.
Thanks for the comment.
To my way of thinking the vibrations resulting from the repairs may have contributed, but then it would seem your average traffic would cause vibrations also.
Along those lines, harmonics is an interesting field. Perhaps the jackhammer or whatever hit just the pace to cause the worst damage for the failed part.
Take care.