Freezing rudders is a hazard of hauling out for the winter. Unavoidable in some areas. Boats kept in the water avoid this risk. Just a part of the trade off.I'm only speculating that it was freezing. If the rudder is hollow, I don't think delamination would show up like that - would it? Nor would rust "jacking". I suppose it someone foam filled it in the past as poatpoker suggests disintegrating wet foam would expand and do this.
When work gives me a break I'll go drill a drain hole and perhaps put a borescope to work. I'll let you know what I find.