Friedrich Hayek tried to answer this in the 40s, when "intellectuals" were into communism and eugenics (this video is not from the 40s obviously.)
http://cafehayek.com/2014/10/hayek-on-intellectuals-and-soci...
To be frank, I think it's because those who are highly educated often believe they know what's better for a person than the person does; economically & politically speaking.
In terms of acceptance of minorities, especially (gay, trans or just ethnic) I think that liberal mindset comes from an academic need to include minority opinions into discussion.
I do think there is a bit of a shift happening and that's many professors, etc standing up against "safe spaces" on campus. The irony of course is that many young people vouching for these areas often identify as liberal, but yet suppressing the ideas of others is anything but. So...you really have to be careful in how you define "liberal," as it appears the title is shifting a bit due to millennial deposition.