In all the information I have found, one thing seems to be clear - nobody knows definitively why these birds do this. Lack of condition and poor health or too many disturbances to the breeding pair are a few of the suggested causes. One theory states that the parents may be startled by the newly hatching eggs, triggering an “intruder alert” and that the babies are tossed because of territorial instinct.  For whatever reason, it can be profoundly heartbreaking.

 
I have had some luck returning pitched babies to the nest.  This is not recommended by most breeders as it has an improbable outcome.  I had one pair that tossed the first two chicks from every clutch, but took them back if I returned them and continued to raise them.  I have had some luck by removing the male of a pair and leaving the chicks to be raised alone by the hen.  Other times, the hen would abandon the chicks when the male was removed.  Sometimes they only toss one or two - sometimes the whole brood.

If the timing is right, fostering the tossed chicks with another pair is usually the best way to go.  Always note the lay dates of your eggs so that you can watch for tossed babies on hatch days.  They don’t survive long on the floor of the cage.