Why and how do hemangiomas form within the vertebrae?

In reviewing 5 MRI's over the past 2 1/2 years (4 of my lower back and 1 Thoracic), my family doctor and osteopath noticed that each time I had a new hemangioma at a new level which the radiologist have said do not need further investigation, although they were not comparing the MRI's. Why or how would these hemangiomas develop? Wouldn't it be significant if new ones keep developing? I understand that they do not have any impact on my sciatica or degenerative disk disease and are not serious in themselves but it has bugged me that they form at all without injury. I would appreciate your input. Thank you, dockside