I thought Kasumigaoka enjoyed being with Tomoya? And Sawamura was his childhood friend...
Originally; both were content just to be with Tomoya, but after being scouted out by professionals far more experienced than them (in the last arc); they realized that they had more to go in terms of experience/skill.
Adding to the fact that a rare opportunity to work in your desired field (being scouted no less) is just too good of an opportunity to pass on.
This is one of the reasons why I liked the second season, it's because the theme: "We can't stay the same forever" or that "Nothing lasts" was very well executed.
In summary: Katsumigaoka, and Sawamura felt that their skills won't improve any further by being with Tomoya, so they left in an effort to improve their skills so they can make even better games with Tomoya in the future.