zxcvbob said,
"But more likely the radio just did a scan for some reason and found the next strong station."
That's what I said above. OP probably drove through a dip* in the weaker station's signal and the radio captured the stronger one with its automatic tuning.
Terry, 230RN
* Due to any number of things, some weird reflection, local change of polarization of the signal, blockage by some object (truck, building, advertising sign), interference by its own skywave (~0640 CT), et, as they say, cetera.
Edited to correct "zxcvbpb."