Understanding the Wave times?

no this is NOT Steelhead specific just try to see how they line up in general. is it according to amount of people on the waves or the speed?
For example Accenture let go 35-39 year old males fairly up front last year but NYC let them last.
So what is the logic lies behind the wave starts? Do they let go the slowest people first so they will open the roads faster top traffic?
Or they are really concern about the quality and safety of the race so they let go faster age groups first and slower ones last to avoid any congestion?