I have a specialized hardrock, and jsut recently my crank on the left side continually loosens and then falls off. It's never happened before, and I tightened it super tight after it fell off the first time, and then it came off again, after about 20 minutes of biking. I took it to the shop today and had them take a look at it, I rode it for a bit after and it seemed fine, but I am still a bit worried. Anyone know what could have gone wrong?