
I would guess that it probably has something to do with the ease of which
ideas and thoughts are communicated on a college campus.  In the real world
(tm) it's easier for theists (well, people in general really) to lock
themselves into a little bubble where they only see and talk to those
people who are of the same opinion as they are.  In college you are
constantly surrounded by and have to interact with people who have
different ideas about life, the universe, and everything.  It is much much
harder to build a bubble around yourself to keep everyone else's ideas from
reaching you.

So, in a world where theists are forced to contend with and listen to
atheists and theists of other religions some are bound to have a change in
their beliefs over four years.  There is nowhere to run.... :-)
