Panic was such a strong emotion. It takes over everything even your state of mind. This is what unknowingly brings her to the Commons.
Sara had wanted to swim back to the Harbor to find her mom (not knowing her mother had just traveled back to the Inlet) and instead reaches the main island. She knows the swim had been longer, but she couldn't go any further without rest. Had they rested on their journey last time? She swore they had so she assumes the place she came to was safe.
At first the terrain felt a little rocky and uneven so she travels a bit further to the west (thinking she was going east) for a more flat surface to lay on. The area didn't get any better so she finally just picks a spot for a nap. Unknowingly she was towards the upper edge of the Peak with the sea close by. She could smell it and she kept it that way because she knew she would need to reenter the sea to get to Atlantis.
She wakes some time in the morning not realizing just how tired she had been. By now she had been gone almost 24 hours from the Inlet. Gone from the lovers she left behind. The red mare enters the sea at the closest point she could find and angles herself north. When she finds land again she does not realize it was the shore of the Commons.
Having gone too far west she ended up swimming down from the edge of the Peak right to the very land she should have been avoiding. Sara shakes her cost free of the water and begins to go a little inland to the north, but then south. She kind of remembered having to go south once she reached Atlantis to find the Harbor. "Adventure here we come ♪♪," She sings out loud as she walks. "To the jungle to my mom♫." Her head bobs back and forth with each note.