Hi,
It’s not understood why this happens sometimes in captivity. It’s more common in budgies and cockatiels and cockatoos. Possibly the presence of the grown offspring is causing a problem. It’s not natural for the offspring to remain with or near the parents once they are fully weaned. In the wild, the parents would chase them off to find their own way. This is nature’s way of preventing inbreeding, since related birds should not be bred. If you have all of these birds in a cage or flight together, that’s fine as long as they get along, but never give them any type of nest and discard any eggs that are laid. I would be cautious having the male with the grown offspring – he could turn on them at any point. If the offspring were not in the same house, the pair might be compatible again.
Thank you for asking Lafeber,
Brenda