This is caused by a nearby transmitter with a strong signal on a close frequency, which overloads the receiver and makes it unable to fully receive the desired signal.
[1] Typical receiver operation is such that the Minimum Detectable Signal (MDS) level is determined by the thermal noise of its electronic components.
When these spurious signals have a power level that is higher than the thermal noise floor, then the receiver is desensitized.
It is these spurious signals that degrade the ability of the receiver by raising the MDS.
Consider the case of a repeater station, a station consisting of a transmitter and receiver, both operating at the same time, but on separate frequencies, and in some cases, separate antennas.