Here's an excerpt from SI on blood clots & athletes:
Professional athletes are not unique in developing DVT/PE. It?s estimated that half a million people are affected by the condition in the United States each year, and it?s relatively common in the octogenarian population. But there are certain risk factors that feature prominently in the pro athlete?s lifestyle: namely injury, immobilization, dehydration and long-distance travel (especially by air).
In contact sports like hockey, hard hits can cause trauma, large and small, internally. And then sitting on long, cross-country flights?particularly after games when dehydration causes the blood to thicken?inhibits circulation, often causing blood to pool in the legs, where platelets may gather and form a clot. It?s not common, but given the constant travel of professional athletes, their chances of developing a clot are heightened.
Often, though, blood clots in athletes stem from surgery and the subsequent immobilization. Varejao developed a clot shortly after surgery on his right quadriceps; Dupuis?s initial episode with DVT came after ACL reconstruction last December.
?[Undergoing] any of these surgeries?like joint surgeries, where you?re going to be laid up for a period of time and you?re not able to use your legs like you normally would?does predispose you to getting these clots,? says Dr. Peter Vitanzo, a sports medicine specialist with the Rothman Institute in Philadelphia.
Now, take those outside risk factors inherent in an athlete?s life and add a genetic condition that predisposes a person to abnormal blood clots.
More:
http://www.si.com/edge/2014/12/16/blood-clots-pose-deadly-threat-to-athletes