I always had it down to the seat being so hard, reduces blood flow to veg parts, I get the same when I'm cycling and cycling in extreme cold weather just means sometimes when you need a piss you will just make a mess of yourself as your best friend has dissapeared

edit
googled it! I new I had read something about it way back and not just dreamed up a medical answer to cover for me being so tiny!

http://www.nature.com/ijir/journal/v.../3900929a.html