John,
Sounds very suspicious to me. With a boat that old, it could simply be that 
the rivets broke from fatigue, but I rather doubt it. You will need to get the 
boat on the trailer, remove the rub rail insert (rubber) and do a further 
inspection. It might mean that you didn't have the boat rigged properly, meaning 
to much tension on the standing rigging. The foam insulation in the bow could 
be water logged, putting more pressure on the bow of the boat.
Without seeing the boat, it is really hard to know what happened. Either way, 
she needs to come out to at least replace the rivets.

Rummy