I've included a short blurb from the following link; for more information go to the site:
"...Cause of Lower Extemity Edema of the Leg / Foot
The etiologies of leg edema are multiple are there is a very long list of possibilities. Some of the more common causes include include kidney problems (kidneys are designed to rid your body of excess fluid), circulation diseases such as blood clots or venous valve insufficiency, heart problems such as congestive heart failure or arrhythmias, prolonged immobilization (such as during prolonged travel), surgery, physical trauma, pregnancy, medications such as hormone therapy, obesity, cancer, vein catheters, May-Thurner’s Syndrome, advanced age, lymphatic system damage, and infections...."
As you can see, there's often no simple 'one' answer to these things....