1. Lumbar extension, usually to compensate for the mobile inability to raise arms above the head.
The inability to raise the arms above the head can have several causes, among them tight muscles, structural problems, the rigidity of the thoracic spine, difficulty in full shoulder flexion, etc.
2. Hyperextension of the pelvis and anterior pelvic tilt usually as compensation for rigidity of the thoracic spine which in chain reaction does not allow the arms to rise above the head.
It's important to maintain the natural curves in your spine to prevent lower back strain.