I have swelling on one side of my posterior skull. I had a head injury many years back. I had c-t scan also. Can this swelling develop into tumor?

Sounds unlikely. Soft tissue swelling doesn't usually develop into a malignancy but need to know why you have the swelling. Is it new? You should be examined carefully.
Unlikely. Usually not. Here is some additional reading on risk factors and brain tumors. http://www.cancer.net/cancer-types/brain-tumor/risk-factors.
Unlikely. It is unlikely. If the swelling is getting larger or is getting more painful then it needs to be re-evaluated to make sure that it does not represent a sarcoma or any other kind of soft tissue tumor.