I have asked others and myself these questions without coming to any conclusion.
Some writings state it comes from poor vitamin distribution.
As time went on, even as much as two years, the ultimate result was a gradual wasting away and inability to perch. Heartbreaking as it was, I felt that the birds misery had ended. Particularly regarding finches you don't have much time in turning the ill bird around. After the seven days of treatment it still had the exact symptoms as before. YET the twirling was still present.