The reason your chainplate sheared may be down to a faulty weld - but given the age of the chainplate before it let go, and from my understanding that the bit that gave up was plastered against the deck, it might have failed due to crevice corrosion or chloride stress corrosion which are two really "special" failings that stainless steel suffers from in dark, damp (especially seawater-damp) spaces.
The chainplate for my starboard backstay on my 32' Cape Carib recently let go in a similar (though much less dramatic) way. I noticed it that the cotter pin was about 1/2" further above the deck than on the port side, and a firm tug revealed why.
I've since replaced both backstay chainplates (the port-side one looked okay from a visual inspection, but I didn't choose to chance it) as well as taking off the bow roller/stem/forestay fitting and checking that out, and I plan to remove the chainplates for the upper and both sets of lower shrouds in due course to inspect them.
Anyhoo - bear it in mind - check out http://www.corrosion-doctors.org/Mat...stainsteel.htm
Alternatively, one of these other egg-heads might be able to give you the lowdown and dirty on it.