LOL! It probably has more to do with the predominately teenage local crowd that begins to get overwhelming by mid afternoon than anything else.
They bought their AP's using the monthly pay plan, they've paid for their food for the entire year (lunch and dinner on each park visit) for $75, and they get free tickets to bring their friends.
I think Six Flags figured out there's a ton of disposable income in that demographic, they just need more of them in the parks. They don't seem to put as much line pressure in the parks per-person because they are content to just hang with their friends.
So they attract and use one group of visitors to create higher guest turnover per day among the rest of the groups.