Technically, the fence does not "cause" interference, as in generating unwanted frequencies out of thin air. What it can do is reflect energy off of it. The phase angle and magnitude of that reflection can vary dramatically with a small frequency change. So if you are trying to send a widband signal with some sort of complex modulation format, as the signal takes up a wide bandwidth somewhere in that band the fence might add in a multipath drop out. that drop-out in phase or amplitude can cause bit errors, and if they get big enough, the network either goes down, or has to remodulate itself at a much lower data rate to live with the multipath interference.