Adding hops introduces a ton of nucleation sites - places where bubbles can form. CO2 can easily come out of solution when there are more nucleation sites, because it becomes easier for the tiny bubbles to form. It's not necessarily yeast activity.
This is also the reason you're not supposed to microwave water in a very clean vessel to boil it. It has no nucleation sites (or very few) so it can actually super-heat and then boil in one volcanic action when you go to take it out. I have to put a chopstick in my 2L flask when I boil on the stove with it or it will do the same thing.
Yeast activity can degrade dry hop additions by both carrying the aroma away with the CO2, and by binding hop oils and trapping them as they flocculate. Then again, they'll also scrub out any oxygen that is introduced when you dry hop if they're still active, so some people prefer to dry hop with 10 gravity points still to go.
