Patrick,
You are right, the boat is stored outside with rudders attached, and yes there probably is a high UV index, but there is another rudder some 7 1/2 feet beside the one pictured with no trailing edge chipping. It looks to me like the outer laminations are not well bonded and the extream pressures and vacume on that edge may be enough to pull pieces of resin / glass loose. I don't know if that is possible or not, but if that is the case, wouldn't that require a different type of repair?
Will Lints