The recent Star Wars Battlefront alpha contained quite a few details about how the Walker Assault game mode works that you probably missed.
The PC alpha gave players some insight into how the game progresses, but only if they weren’t distracted by how gorgeous everything looked. YouTube channel BattlefrontUpdates went through that footage to try and find out how you actually win and what you need to do.
The video below covers the issue in more detail, but it’s somewhat similar to Rush from the Battlefield series. One team tries to capture uplinks, only two of which are active at any one time. When that’s done, a Y-Wing gets added to reinforcements to bring down the shields of the AT-AT for Rebels to shoot it.
Keep in mind though that while this is based on the alpha, the final game could be different.