IF the holes the fasteners went through weren't sealed, then water could have easily entered the laminate through the raw fiberglass, by wicking along the glass fibers there. Then small amounts of water freezing and thawing could have caused the damage to the gelcoat you're seeing.
I did think about that.
But if that is the case, I would expect damage to be radiating in circle from the holes, as water wicks pretty much evenly (or perhaps slightly more downward). Instead, it is specifically limited to exactly the area under the ground plate. Since distance from holes to middle of plate (or to vertcal edges) is more than distance to top and bottom edges, if damage would have radiated that way I'd expect the circle of damage to protrude up and down from the holes beyond the immediate ground plate square. It does not. Also there is more damage in some areas farther away from the holes (but still under the ground plate) than it is in their immediate vicinity.
The fact that it is so precisely limited to ground plate shape really seems to invalidate that theory (at least not by itself, may be as a contributing factor).