Answer:
Injury to the vain.
Step-by-step explanation:
Thrombosis is affected by virchows triad of stasis, turbulence and coagualability of the blood. An increase in any of these three can cause thrombosis.
The blood pools in the legs when we stand or sit and we very rarely are in a situation where gravity aids in draining blood from the legs. The body forces the blood up the legs by squeezing it up when we contract our leg muscles. We have one way valves in these veins to prevent the blood from falling back down the vein. So if we are in a situation where we are vertical for long periods of time or not moving our muscles or have had a previous thrombosis that has damaged our venous valves this can further increase the stasis already present in the legs and precipitate a thrombosis. It is this predisposition to stasis that causes dost thrombosis to occur in the legs.
Flying causes DVTs for this reason as we are sitting still for long periods of time. Also the recycled air can cause dehydration which increases the coagulability of the blood.