Bicuspid Aortic Valve Cause

My echocardiagram results showed that the aortic valve had three leaflets but that two of the leaflets were fused, leaving a functional bicuspid valve with prolapse and moderate leakage.

My question is how did these leaflets fuse?  What causes them to fuse?  It is my understanding that the aortic valve can be damaged from a congenital defect or as a result of rheumatic fever.  Based on this report, I was born with a healthy, functional valve, but somewhere along the way it became altered.  I don't ever recall having rheumatic fever either.  Are their any other causes that could attribute to two leaflets fusing together?

Matt