The cause for Paget's is a bit vague. Earlier it was thought to be an inflammatory disease. In fact, the person after whom the disease is named, Paget himself believed that the condition is inflammatory. Later, it was theorized that Paget's is a circulatory disturbance, because the bone in this condition becomes excessively vascular. In other words, the bone receives blood supply in excess of its requirements. There have been suggestions that the alteration in the bones is similar to the development of arteriovenus aneurysms.
Another cause suggested by a scientist named Jaffe is that there is a failure in the mechanism that replaces the bone constantly through a process called creeping replacement. Again there have been pros and cons, and this theory does not hold much water. Finally, it has been theorized that Paget's is caused by a slow virus, the origins of which are as yet unknown. Ultimately the cause of the disease still remains a mystery.