Several things. Dvt occurs when virchow's triad is satisfied: hemodynamic abnormality (stasis, turbulence) endothelial damage, and hypercoagulability (abnormality in blood due to malignancy, increased risk in clot formation). Family history (thrombophilia), surgery, trauma, dehydration, prolonged bed rest, congestive heart failure, kidney problems are risk factors.
Many things. Risk factors for deep vein thrombosis are numerous. One of the most common is immobility. This can be caused by other medical problems, surgery, or other issues. Certain surgeries are higher risk than others. For example, orthopedic and bariatric surgery. Traveling long distances in cars or airplanes are another cause of immobility. Underlying malignancy or hypercoag state can also cause dvt.