Why do the birds I feed bite?

I feed wild sulphur crested cockatoos every morning and night. They come and sit on my windowsill and wait for their food. They have become quite tame and let me pat them and they come onto my arm. Sometimes when I try to feed 2 at once one of them bites me. My husband thinks it is because of the “pecking order.” Do you agree or do you think it could be because they are just being greedy? There is plenty of food for all of them.