Classic case of EMC - Electro Magnetic Compatability. Some non-linearity somewhere (could be a dirty connector) or a lack of or failure of cable screening allows the radiated signal from the aircraft transmitter to be detected. Many audio amplifiers will detect radio, either because of a large radio signal exceeding the capability of the amplifier or by bad design, or sometimes both. Happens with telephone exchanges too - a PABX about three mile from the Brookman's Park BBC station always had Radio 4 in the background, and it probably wasn't the only one to suffer. Telephones are bad, too. Way back in the mid 1980s, they put new electret microphones in telephones at Manchester airport: they detected all the air traffic signals, and had to be replaced with the old carbon microphones!
Unfortunately, even these days, designers rarely pay enough attention to EMC considerations at the design stage, and then it costs a lot more to sort them out when design is almost finished.