10 day old cockatiel died, can you tell me why?

by Pamela
(Rhode Island)

My tiels laid 5 eggs, 2 hatched. The first baby to hatch the parents took care of, fed, brooded and the baby was growing fine. I had to pull the second baby to handfeed at 2 days old because they abandoned the little one.
The one that I have been handfeeding is now 8 day sold and seems to be doing fine.
This morning I checked the next box with the parent raised baby and everything was fine, baby looked fine, he was walking around the nest box trying to flap his wings crying for food and mama came and fed him. Later in the day I opened the nest box and the baby's crop was blown up like a balloon he was laying on his side twitching making strange peeping sounds and died very shortly thereafter.
Can you tell me what happened?? Is there anything I can do to prevent this from happening to the baby I am handfeeding? I am horrified that this happened.
Thank You
Pam