I don't think we discussed politics much at all in our grad student offices, mostly football, other sports, silly things like creationism and other pseudosciences. As for in the classroom, there was exactly *ZERO* discussion of politics in my graduate work at a public university not from the professors, nor from the TA (me). I did go from a satisfied member of the Church to one who was so dissatisfied I would leave after a few more months. IIRC PhD holders are significantly less religious. I suspect that phenomenon is related.
Here are things I have noticed that might be relevant:
1. Unlike BS/BA programs where most attend in their home state (as you and I did), most graduate students travel out of state for their degrees (as I did). That exposes you to other cultures (like evangelicalism in my case) and at the same time keeps you from tight maintenance of tight reinforcement of your home culture. (I went home once a month in undergrad and worked in my hometown in the summer. In gradschool I mostly went home at the end of each semester for a week or two and stayed on campus in the summer.)
2. It is a lot easier to complete a BS/BA with out acquiring reasoning skills than a masters degree and almost impossible in a PhD program.
3. Both religious positions and conservative ones tend to thrive in the "not questioning" anything mode of thinking. Being well traveled, living away from your home/culture, and developing strong reasoning skills are corrosive to maintenance of religious and conservative positions.
You're point on number 1 is interesting, and admittedly I hadn't initially considered that.
However, when you mention that there wasn't much political talk when you were in college, that was was my experience as well, but the polling I linked would indicate that the environment may have changed a bit.
54% of students say some or many of their professors express their political views in class
Results like this:
When asked if they have professors who talk politics during class, 32% of respondents said that they have a few professors who do, 41% say they have some professors who do, and 13% say they have many professors who express their political opinions in class. Of these 85%, two-thirds say their professors occasionally (52%) or frequently (15%) express these ideas.
I don't know that I could tell you any of the professors' politics from when I was in college (I mean, you can make guesses, but they never went out of their way to discuss it). Even in a Poli-Sci elective class, the instructor in that course obviously talked politics in a general educational sense, but I don't know that they ever overtly made clear which "side" they were on, on the various issues.
The fact 85% of students are reporting that their professors are "getting political" marks a shift away from the college environment you or I were in. (assuming that we were in college around the same time)
I would've been in that circled demographic, during the time where the biggest share of the pie was still the "mixed" category. (even among the post grads in 2004, the "mixed" group is still the biggest.
The only social catalyst I can think of during that time period would've perhaps been that Bush 2 was a bit of tumultuous presidency (although, people seem to see him as "one of the good ones" in retrospect), and I do recall in the mid-2000's, there was a certain "coolness" associated with being Anti-Bush among some of the younger crowd.
For those who remember these that came out in late-2004:
(and it was actually a pretty
star-studded line-up on that)
But my time in college was drawing to a close for that period...so who knows, maybe 2006 would've been a very different environment on campus.