I would think that if it were from an impact there would be some other damage on either the rub rail or the cap rail.. If water leaked into the chainplates at all in the past (I see a lot of silicon around them in the first photo) it is likely that they are going to have crevasse corrosion given their age. Just because chain plates are glassed in does not make them impervious to leaks, it just makes it harder to tell if they have been leaking. In this case it would not have to have had that much force to cause a failure, certainly not enough to deform the deck. The cracking looks to be in the area where the chain plates would be welded to a cross bar and the weld would be the most likely area to develop crevasse corrosion as it would naturally have a lot of nooks and crannies. This is typical of 20-30 year old ss that has lived in a marine environment.
When we pulled our plates this is what we found.. On all of them.. It was because of this that I have recently started (1 year so far) doing rigging inspections under the supervision and along side of a local surveyor. While I don't have 30+ years of experience under my belt, I have inspected quite a few rigs. There hasn't been a single one that I have looked at that was older than 25 years with original chainplates that did not have at least one with cracks (this isnt to say that they dont exist, I just haven't come across them yet) If it were me with a 20+ year old boat, with glassed in chain plates, and signs of spider cracking on the hull over one of those plates, I would sit down, pour my self a tall glass of rum, and break out the hole saw.