There is no such tradition. If you read anything before the 1930s, the universities are pictured as the bastions of conservatism, often of the worst kind. This "tradition" is recent in origin; specifically, it is post-war. The question is what happened in the early 1950s to change the character of the academia. Ideas 1 to 2 are entertaining. Idea 3 is the unfortunate reality, but it is the consequence rather than the reason. If you look at the data in the paper, 25% of American sociologists are Marxists. I mean this literally; that is how they define themselves. Yes, it is a given that they will hire other Marxists; the situation reached the point when the propagation of Marxist sociologists is self-perpetuating. Interestingly, Gross gives idea 2 as the explanation of professorial leftism. According to him, sociologists are far to the left of computer scientists because sociologists are smarter. He means it in all seriousness, thus convincingly proving the opposite.
Actually, I do not know the answer, if you discount various nonsense. Surprisingly, it is difficult to find any credible research on this topic (and I've looked). My hunch is that it is an echo of the GI bill and certain other factors. Absorbing a huge mass of students required the expansion of the higher education system. A lot of professors were drafted into the war effort. Quite a few of them did not return, either not coming from the war or finding more attractive jobs. At the same time the administration had changed and two generations of New Dealers found themselves without sinecures and the prospect of employment; nobody needed them in the US. These people were absorbed into the rapidly expanding academia, especially liberal arts colleges, quickly dominating the humanities. This generation-1 educated and trained the first wave of baby boomers that became generation-2. Then it became self-perpetuating. Quite simply, it became impossible to be educated in the humanities (and all student need to take such courses) without being exposed to (at least some) leftist brainwashing. The diffusion from there to public schools made this exposure even earlier which assists indoctrination higher up. By generation-3 and -4 the system became self-perpetuating. So the reason is WWII and its immediate aftermath creating the unique opportunity.
Re: среди профессоров преобладают ...
Actually, I do not know the answer, if you discount various nonsense. Surprisingly, it is difficult to find any credible research on this topic (and I've looked). My hunch is that it is an echo of the GI bill and certain other factors. Absorbing a huge mass of students required the expansion of the higher education system. A lot of professors were drafted into the war effort. Quite a few of them did not return, either not coming from the war or finding more attractive jobs. At the same time the administration had changed and two generations of New Dealers found themselves without sinecures and the prospect of employment; nobody needed them in the US. These people were absorbed into the rapidly expanding academia, especially liberal arts colleges, quickly dominating the humanities. This generation-1 educated and trained the first wave of baby boomers that became generation-2. Then it became self-perpetuating. Quite simply, it became impossible to be educated in the humanities (and all student need to take such courses) without being exposed to (at least some) leftist brainwashing. The diffusion from there to public schools made this exposure even earlier which assists indoctrination higher up. By generation-3 and -4 the system became self-perpetuating. So the reason is WWII and its immediate aftermath creating the unique opportunity.