Pick order based on..
I tried google and couldn't find anything, so I decided to put it here.
It really seems to be based simply on..who accepts into the match in what order.
Everytime I joined at the last second(ish), I was always last pick on red. Likewise, hitting at as fast as possible yielded a pick number of 1-3.
Maybe needs more trial, and maybe shouldn't post here, as not fully confirmed.
Definitely not based on mmr, I've seen dodged games with the same 4 people in the next lobby, totally different pick order. Can't be it.