This is likely tropospheric ducting or E-skip.
Tropospheric ducting ("tropo") happens when there is a high pressure system nearby and the warm weather forms above a cooler layer of weather. In other words, a temperature inversion.
E-Skip happens when there is a build-up of ionized particles on the E-layer of the ionosphere due to sunspot activity.
Both phoenomena will result in VHF signals within the FM band to travel for hundreds and in some cases, thousands of miles. The lower the frequency, the more prevelant this is. In the days of analog TV, there have been many reports over the years of reception of Central American and even some South American TV stations operating on Channel 2 (54~60 MHz).
Tropo is the reason why KGB-FM San Diego comes very clearly into Los Angeles on 101.5.
LPFM and FM translator stations are more vulnerable to the effects of tropo and E-skip as they do not have the signal strengths to try to fight it. This is why it may seem like another station is taking over your frequency.
There is no need to change channel as this is a sporadic condition and is seasonal in nature. For LPFM stations, the FCC will not permit a non-adjacent channel change under ยง73.870(a) of the rules if the sole reason is tropo or E-skip.
