Tower Frequency Changes


I can’t seem to find an answer to this question: Why do towers suddenly change to new frequencies? Case in point, I lived in Palmdale, CA for several years where the tower frequency at KPMD was 119.2. Suddenly one week it changed to 123.7. Similar situation at nearby William J. Fox (KWJF). Their tower had been 120.3 for many years, now recently it was changed to 118.525, which is a frequency normally delegated to ASOS. I don’t see any conflicts with other frequencies in the area. Just curious as I currently live in San Diego and KSAN tower has been 118.3 for decades. Thanks!