Jim...it would be helpful to understand the type of boat and nature of the failure. But sailing dawg is right about the tremendous forces that can be exerted in reverse or by wave action.
One possibility is water seepage into the rudder foam/laminate and then attacking the stainless post/grid which can corrode quickly in anerobic conditions causing failure. Delamination is another possibility especially if there was water in the rudder and it was stored on land in freezing conditions during the winter. If either of these conditions existed, then just a bit of heavy pressure on the rudder could have broken things loose.