I’ve heard a convincing theory that they function as a sort of “sperm” seeding the development of an apothecium in another thallus. (I’ve forgotten which hyphae have which complement of chromosomes!) And frankly, I’d be surprised if it were the same across fungi, or even across all the independent origins of lichens.
I bet there was a juicy bird splat in this particular spot on that perfect day some many years ago, when sun, moisture, temperature, everything was just right. Ascospores are touching down on all surfaces regularly; I bet thallus initiation is limited not by the chance encounter of a spore, but rather by the precise conditions on the surface.
There’s an anecdotal story attributed to Yngvar Gauslaa. He inoculated a patch of bark with Sticta fuliginosa(?) one year. He revisited the tree repeatedly over the years. Nothing happened. He would return less and less frequently, until one year after he’d all but forgotten about the “failed” experiment: There, all of a sudden, are dozens of tiny little Sticta thalli. Our interpretation was that presence of diaspores (isidia in his case) is insufficient. Somehow they hung out there and survived for years. Then one day conditions are just right, and they all simultaneously establish and start to grow out into full-blown thalli.
So maybe over the years, spores of all sorts of different species were collecting on your seaside rock. Then one year the conditions are just so for this species, and all the spores (three in this case) “wake up” and this is what results.