I don't know if anyone really knows the cause but people with pathologic or progressive myopic degeneration may see some progression even after age 25. You should make sure you are being seen by an ophthalmologist and retina specialist who can keep tabs on this and make sure your eyes stay as healthy as possible. How high is your prescription?
I not only see an ophthalmologist every year, but I have been seeing a retinal specialist because I had a myopic CNV in my left eye. He has now given me two injections of Avastin, but although the swelling went down, my BCVA has not improved since the incident.
I still am trying to figure out what could cause the progression if the eye is not getting longer. I realize the retina gets thinner, but I though that that was caused by the constant stretching of the eye.