If you watch Dr. Oz, you heard him tell a woman that there are actually a few causes of bags under your eyes.
1. Allegens. (Those itchy eyes!)
2. Sleep position
I have an easy time understanding how salt and the allergens can cause problems. But I'm not sure I believe the sleep position thing.
When you have allergies, your eyes might itch and tear. Of course they will be puffy.
When you ingest salt, your kidneys can only process so much so fast. What the kidneys cannot process immediately gets stored under the eyes, and in the hands and feet. If you don't sweat it out pee it out, it will remain in your body for longer.
When you fall asleep at night, you might end up puffy in the morning because your kidneys work more slowly at night. I wake up puffy no matter what position I sleep in if I had too much salt in my food.