Its more likely in an aircraft because of dehumidified air that comes into the cabin at high altitude and of course reduced amount of oxygen...thus the blood becomes thicker because of it....then if you don't move for quite some time, then there is a high probability a clot will form....
Even at the blood bank, they keep the blood slowly moving so as to prevent clotting, as the doctor told me if blood is stationary it will clot, then if you are predisposed to clotting, then you could be in serious trouble....so the compression socks will help blood from pooling in your legs and thus maybe prevent at DVT forming.