Two reasons - increased skin blood flow and gravity. It can be uncomfortable, especially in hot weather, but isn't anything to worry about. Short of running with your arms above your head, there's not much you can do to prevent it.
If your hands actually become painful or partly numb when they swell, you may be developing a condition called carpal tunnel syndrome, where the thick gristly band that holds all the nerves, tendons and blood vesseld together in your wrist becomes thickened and presses on a nerve. If this were the case, I'd expect you to have sore tingling hands when you wake up in the morning too. This would be a reason to see your doctor.
Cheers, V-rap.
