As above - it's a normal part of that mission.
When you enter the mission, there is a countdown during which the game waits for a team to enter from the other Quarter. If there is a team, the countdown can be shorter.
If no other team enters before the countdown ends, then the other team will be made entirely of random henchmen.
It is sort of possible to co-ordinate this. If you know another team/player who wants to do the other Quarter at the same time, you can co-ordinate your timing by chat or Ventrilo, etc, to enter at the same time. That used to be difficult because there were a lot of teams entering and the choices were basically random, but these days, you have a better chance because the game will take human players over all-henchmen teams.