If anything, male college enrollment in the U.S. is still relatively high compared to some countries in Europe. The best explanation I have seen is that the trends are an outcome of the continuing gender gap in reading and writing in elementary and high schools. Boys lag way behind girls in these subjects, not just in the US, but world-wide. As a result, they get lower grades than girls. The effects of that campaign are especially obvious in graduate and professional school enrollments, where women are rapidly approaching parity with men.
Many states publish similar statistics and all show the same trends.
The most convincing one is that it stems from the virulently anti-intellectual schoolboy culture. It doesn't affect all groups equally. The sons of east Asian immigrants are not underrepresented in American colleges. For the several decades before widespread assimilation, neither were the sons of American Jews. These two groups have strong traditions of veneration of scholarship that inoculate their boys against the prevailing culture.