First, if something repeatedly happens on a part that was previously working then one would have to conclude that it is your action that caused it. Over-tightening would by far be the most likely cause.
Unlike bolts that secure parts in place and prevent rotation, the cantilever pivot bolts have purpose but to keep the arms from coming off while allowing free rotation. Therefore the bolt should never be tightened down too much, as it will cause the bushing (the brass sleeve around the post) to mushroom or bulge outward, causing binding. The proper tightening is just enough to hold it in position against the end of the bushing. Once the bushing is damaged it takes very little pressure to cause the problem. I think Andy's suggestion of thread locker, as well as more care in the future, is a good solution.
You just need to carefully sand down the bushing until the arm works smoothly when the bolt is secure.
Last edited by cny-bikeman; 05-31-13 at 07:27 AM.