Why A receiver's local oscillator is never a pure single frequency; its output carries phase noise skirts on either side of the carrier. When a strong signal appears near the frequency you are listening to, that signal mixes with the LO's noise sidebands and the noise gets translated right into the IF passband, raising the noise floor and masking the weak signal you want. This is called reciprocal mixing because the roles are reversed: the unwanted signal mixes with LO noise rather than the LO carrier. A cleaner, lower phase noise oscillator is the cure, since no amount of IF filtering can remove noise that is already inside the passband.
Watch out Two out-of-band signals combining to make an in-band product describes intermodulation distortion, a non-linearity problem in the mixer or front end, not a phase noise problem in the oscillator.
Reciprocal mixing = LO phase noise plus a strong neighbor. Fix the oscillator, not the filter.