Radar signals have a thing called 'side lobes'. Without getting too much into detail, the radar beam not only emits where the dish is pointing, but also off angle as well. This energy can create reflections against things the dish is not directly pointing at, resulting in interference.
In the case of the Sapfir, it is likely that at altitudes below 3km, the sidelobes are bouncing fairly constantly off the ground, essentially self-jamming the radar. New radars use post processing tricks to get around this, but given the age of the Sapfir, it is something the MiG just needs to live with.