When you are sitting down for long periods of time, such as in an aircraft on a flight or in a coach or car on a long journey, you don't move your legs as much as usual and so the flow is reduced (see diagram on right).

A change in the flow of the blood in the veins is one of Virchow's Triad. 

This reduction of flow allows clot to build up on the wall - the start of a  deep vein thrombosis (DVT) - (see diagram on the left).

In this diagram you can see that as the layers of thrombus start forming on the vein walls. If nothing changes, more blood clots and the thrombus continues to grow. As it grows it slowly blocks off the vein.

There comes a point where the whole vein blocks off - called an "occlusive DVT". 

At this point, there is no blood flow in the vein passed the thrombus, and the clot "propagates" - ie. The stationary blood in the vein also clots.