Why does my bathroom exhaust fan drip during the summer?
I think Chris is right, warm moist air from outside is coming into the vent and condensing on the cooler inside surfaces.

Fixing this could be as easy as fixing or replacing the outside vent damper so it closes properly.  You could also go for a vent damper that installs within the vent itself.  And finally, you can insulate the pipe so that the vent pipe stays close to the temperature of the air flowing through it rather than the inside or outside of the home (depending on where the pipe runs).