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What most happened is 'fatigue failure' wherein the metal simply 'became tired'. Stainless steel is especially prone to work hardening or fatigue failure ... very common in boat design: spade rudders, rigging, etc.
When your boat was designed (70s & 80s) fatigue failure was little understood and the design 'safety factor' that was selected was probably underdesigned, basing the stress calculations solely on 'ductile failure' of stainless @ 90,000 psi. Fatigue failure for stainless needs to be designed at less than 30,000 psi or less to extend the functional service life of the material.
Once fatigue cracks (microscopic fatigue cracks) begin to form water can enter the cracks and cause a second means of failure: crevice corrosion; so, in boat design you get fatigue (because of 'underdesign') and crevice corrosion all at the same time.
Worse, a spade rudder is known as a cantilever (think of flag pole) and the stress design on a cantilever inherently causes the material, where its constrained at the 'root' of the cantilever to be 1/4 as strong as if it were a beam that is supported at both ends.
300 series stainless has a stress value of 90,000 psi; however, its fatigue endurance limit is ~30,000 psi. Requiring that the designer 'derate' the metal to 30,000 psi or less.
All things in nature eventually wear out or get tired, then they fail. So, probably your overstressing your rudder probably only accelerated the failure by a wee bit as especiallly spade rudders will ALL eventually fail ... a design problem, 'safety factor' problem and a materials problem, a 'cantilever' problem and especially when 300 series stainless steel is used. Your rudder simply 'got tired', developed fatigue/chemical failure ... and you were using it long after it should have been replaced. Periodic inspection is the only way to not experience 'most' of such failure.