Eggs are almost never on the clean live coral. frag plugs, live rock, dead coral... all can host eggs and other pests like unwanted algae. I would re-frag anything I put in my tank(not that I would put anything new in anyways) :)
I think it's because they are inbred. They are so clueless when they are laying eggs, half of the eggs get broadcasted to the surrounding area then become food for the other fish :)
you might want to look with some form of magnification. I find it next to impossible that your sand is void of anything larger than bacteria. most of the sand bed critters are not visible to the naked eye.