I've read this article by the New York Times. It isn't substantial and would need to read the books mentioned in the article but it is a theory.

There was also On May 15, as Wallace campaigned in Maryland, a state he was expected to win, he was shot four times by 21-year-old Arthur Bremer. Wallace survived, but he was paralyzed from the waist down. Maryland’s voters surged out on election day to give Wallace a huge victory, his last of 1972. Wallace received many visits from Democratic and Republican politicians, and many wondered if he would endorse anyone else for the presidency. While he recuperated, many of his supporters began to move in overwhelming proportions behind Nixon’s candidacy.

It also points this out: "McGovern’s rise made many Democrats nervous. Some were worried about his antiwar views, while others thought that he went against traditional Democratic principles. For many, unfairly or not, McGovern came to symbolize a candidacy of radical children, rioters, marijuana smokers, draft dodgers, and hippies. With the California primary approaching, Humphrey tried to bring all the objections to McGovern together in a last attempt to save the nomination for himself."