There are a lot of other things that can cause that. Airplane flutter is one. The signal reflected by a plane flying overhead (even at 20,000 feet or so) keeps changing phase, so it keeps reinforcing/interfering with the signal from the tower, making the signal "flutter". That will switch you all over the place in a second or two.
A 55 foot trailer passing near you can cause the same effect. If you're near an interstate, with a lot of truck traffic, you can also get flutter.
And atmospherics can cause the same effect. (If there's a mass coronal ejection on the sun, we can lose all communications for hours [and get some fantastic auroras visible as far south as the southern US]. Smaller ejections can cause all sorts of problems intermittently.)
There are a lot of things we can control, but the laws of physics aren't one of them.