Another possible cause is some kind of skin disease, such as fungal infection, bacterial infection or other inflammatory diseases. These diseases may cause the neck hair loss of parrots.
If this happens to your parrot, I suggest you consult a veterinarian immediately. Professional veterinarians can determine the cause of the problem by checking the bird's physical condition, asking symptoms and conducting relevant tests, and give the correct treatment plan. Don't try to treat yourself, because the wrong method may lead to more serious problems.