Probably from sitting in cramped humvees and tanks and not getting the chance to move around. Exactly. Add getting dehydrated in the desert heat on top up that and the blood becomes even more prone to thrombosis.
In the airline business, this has the nickname "Economy Class Syndrome".
This appears to me (an old man) to be the best guess so far.
My logic may not be right, but here it is.
1-dry air can dry sinuses (an more) and fool one of the actual state of being.
2-dehydration may thicken the blood.
3-this may cause problems for some even without a clot.
4-If it causes a blockage in the small blood vessels it may act like a clot or a clot may then form.
5-The symptoms may be much like a heat stroke.
At any rate be VERY observant concerning water intake and out go.