This is the second time that I have had one of my crank shafts come loose. I want to know why this happens. Both times there has been creaking in by the pedals and then the shaft starts to wiggle as I pedal. When this happened the first time the pedal fell off. My quick fix was to tighten the bolt and keep riding. But it kept getting loose...

I got it fixed (that was about 3+ years ago) and have had no problem with that until now. I don't remember what my lbs said was the problem but it's back. I'm taking it to the lbs over in Kingston later today. I want to know what you guys think causes this problem. All the bikes I've had before this bike have not had the problem.

Thanks

edit> Just realized I can't spell second... ...