It can be caused by not enough oil in the damping side. But as You said You never serviced them so You never poured any oil out so chances are the seal on the rebound damper is letting oil to the lowers. Simple way to check it is to take all the air out and see how much the fork is compressing. If it won’t compress almost fully, leaving just some uppers sticking out than You have too much oil in the lowers and because You never topped up or poured out any oil it could be the oil leaking from the compression damper. Motion Control seal kit is someting like £18, replacing it is pretty easy, but it`s just a simple o-ring, I need to ask some plumbing shops or some other places if You can find just that o-ring that would cost couple pence rather than £18.