I wonder if...physiologically speaking...this has anything to do with the fact that at that very moment, no other sperm can breach the wall of the egg and complete its mission. Only one sperm can fertilize an egg.
It may be so. I recall that tearing the leaf of a live plant created an aura at the tear, that was captured on camera. Thus, perhaps the stress of the egg/wall interaction also creates a similar light. I would like to know how long the light lasts.