...they thought Nixon would end the Vietnam War and they thought Carter was a segregationist?
That and/or they just didn't like his religiosity. I could see an anti-war liberal Republican having this voting history followed by Anderson in 1980 and Democrat in every election thereafter.
Statistically, it is virtually certain there were a few, perhaps more than a few in South Dakota; Lexington and Lincoln, Mass.; Washtenaw County, Michigan; and a few other places. The Left was cool toward Humphrey in 1968, regarding him as part of the "old guard" and much preferring Eugene McCarthy;
The Nation did not endorse Humphrey.
With the exception of South Dakota, all of the above places gave Anderson a big vote in 1980 and have been pretty strongly Democratic since.