Ashton headed over to get his first job as a mercenary. However, at his rank, not many were available in the first place. Those that were available to him, were not available to Anna as she was still stuck at rank 1.
She wanted Ashton to leave on a mission suitable for him, while she would hang back with the twins as their rank was the same. However, ever since the incident with Darjud, he had grown kind of overprotective of her and did not want to leave her alone, but he was not being given a choice.
In the end, he agreed it was for the best to leave her with the twins. But not alone. Sven was to stay within her shadow and protect them if the need arose. This was his compromise. Initially, he wanted to leave Atlas with her, but considering he wasn't fully tamed yet and was unpredictable, Ashton decided to leave Sven instead.