Sorry to intrude on your forum, and condolences to the pilot's friends and family.
As stumpie states, there are underlying conditions, such as his inherited gene, trauma, and forced immobility, which predispose people to dvt formation.
I'm not sure whether the incidence of dvt's in long haul pilots has been demonstrated to be greater than that of joe public, statistically- my more learned colleagues on the medical & health forum may know better than I. Last I heard, there have been some studies commissioned, but I havn't seen any results.
I'm sure I,m preaching to the converted, but the usual measures which cut down the risk of dvt should be encouraged- calf pump exercises, maintain hydration, and avoid alcohol 24 hrs prior to flight is the usual advice to people travelling long haul. Compression stockings can be used for those at risk, but I'm not sure if pilots would fit into this category.
Anyhow, 49 is too young to die.