When electronic components are exposed to signal levels they are not designed for they will go into overload and in a non-liniar mode that makes them work as mixers. Two signals mix and creates a third signal, or several, that only exists in that component. It could be a receiver that are badly designed or use inferior components or the signal strenght are far too high.
Almost everybody will experiance an improved reception quality when using a FM trap filter.
/Ubbe