Not sure if it's CO2 coming out of solution due to all the new nucleation points, or if there's a little fermentation still happening and the bubbles are sticking to the hops, or maybe there's something in the hops that the yeast can eat, but what you see it perfectly normal.
When I dry hop I expect it to foam up a bit in the neck of the carboy, and for the hops to float for a while. One time I added like three ounces to a carboy with not much head room, and the expanding hops tried to push out the stopper. Had to do some quick siphoning to save the day.
Eventually they'll sink, but it can take a while. Weeks.