basicly watter intrusion, then going through hot and cold cycles over the years. the leg, if there is anything wrong, would be from water intrusion into the wood backing rotting it out. dammage can also cause it, but the biggest cause for old bigfoots is sitting in a feild with moss growing on them.
the problem is everyone buys clamshels thinking they are leak proof and don't do any maintenance. the fiberglass still needs to be protected with wax or something, and the seams still need to be inspected a couple times a year and resealed as nessasary