All parents of all animal species other than humans seem to know either instinctually, or through sense of smell or other as yet unidentified method that their offspring, or eggs are healthy or not.
In the case of your birds there may be something wrong with the eggs they tossed, or they were unfertilized, or they felt that they as parents, would be unable to care for the number of eggs in the nest. Since you mentioned that it was the same egg being tossed out repeatedly, it is either unfertilized or there is something wrong with it. Unfertilized eggs weigh the same as fertilized eggs.
I don't know about love birds but in the wild parents will dispose of bad eggs and unhealthy babies. I don't know how they sense these things. At times they will even abandon a whole nest. Over the years we've had young birds brought in by people that found them dumped out of a nest. We've attempted to raise the babies only to find that they would not thrive and grow as they should. Again, I have no idea how the parents know this but they have a good sense.
If you feel the egg was accidentally bumped out, you can put it back in. But after a couple of times I would simply remove that one egg and count it as lost. You could put a small pencil dot at one end of the egg to see if it's the same one.