They are discussing personal issues, such as corruption and greed of parties. Those private issues cannot be discussed in front of the armies since they are secrets and it could stir separation. In a nutshell, they do not want to appear divided in front of their armies.
Brutus and Cassius go into Brutus' tent to argue instead of staying out in the open probably for fear of being heard by someone. They are arguing about Caesar's murder, among other things, and they may not want to be discovered.
