At that depth, the pressure of the rock overhead is just enough for liquid CO2 to be stable, if the temperature is low enough. If you have water cutting these gullies, you should see that everywhere, not just at these specific locations. As it comes out, it expands very quickly, cools, and actually produces CO2 snow. The snow is suspended in CO2 gas that hasn't solidified yet. Together with rock debris, it forms slurry.
Geologists call it a 'suspended flow'.
Suspended flow acts like a liquid. It doesn't take very much liquid each time to add to gully formation. The researchers believe that as the cold wave moves from the surface, the rock pore space is completely filled in. When spring comes, dry ice warms up and expands. Since all the pore space is filled, pressure builds until the ice turns to liquid.