In the early stages of treatment with braces, we use thin, very flexible archwires. The first teeth to respond by moving are the front teeth, because they have smaller roots and need less force to move. The direction in which teeth move most easily is forwards, because that's where the resistance is lowest, so the initial expansion of the dental arch is from front to back. As treatment progresses and we move on to the next archwires, they start to shape the arch at the sides too, and the arch then widens sideways as well. This creates room for the front teeth to be moved further back. And of course, the forward movement of the front teeth can be controlled and limited if it isn't part of the treatment plan.
This answer is for general information. Your own case needs an examination by an orthodontist.