The cause is gravity. It is nothing to worry about. Just fluid settling into your hands as you walk. You should have seen mine after the half marathon walk I did, I've lost a bit of weight so I have a little bit of loose skin around my knuckles, the fluid settled in there and made me look like I had a spare tyre around each of my fingers! It disperses again within a few hours of the walking, it'll help if you hold them up for a few minutes to allow the fluid to drain.