My budgie Spice will often do head bobbing so I researched this a while back. Here are some reasons for head bobbing.
1. Attention getter, wants to come out of cage
2. Really excited about something
3. Head bobbing is part of singing
4. Head bobbing is a symptom of autism (and yes they were refering to budgies)
5. Male head bobs as a sign of affection for female when not mating, courtship behavior
6. Mom head bobs when regurgitating food to assist in regurgitation
7. A motion of love and happiness, even love of own image when looking in mirror
8. If baby, begging for food
9. Budgies move just head in communication as opposed to whole body
10. Just having a fun time