You'd be surprised how much difference there is to reception of broadcast signals due to antenna patterns - do you remember the days of "rabbit ear" antennas on televisions? A tiny difference in location can make or break reception. With the system on, try picking up one cable at a time and moving it a bit - lift up, sideways, etc. I'll bet you'll eventually find that one cable was picking up a station and a move of a few inches will cure the problem.
Any coil of wire can form an induction loop or antenna, and as your wiring is all connected to your gear in some way it's not surprising that this can happen - sometimes radio signals can ride on the ground connections and induce signals into nearby wiring, which is eventually amplified.
I once had a particularly troublesome valve preamp which would detect AM radio all by itself without being connected to anything else! The chassis was acting as an antenna and the signal was being picked up by the slightly microphonic valves I had and being amplified. As soon as the power was turned on to the preamp, even with the power amps off, you could faintly hear radio from the speakers.
Cheers
