"Seojun goes to a party with Seoyeon."
Translation:서준은 서연이랑 파티에 가.
I think its because of the verb 가다. Because you want to go to that particular place you put the location before the verb 가다 and thats also the reason you put location marker 에 after location. For example, if you wanna go to the park you say, " 나는 공원에 가고 싶어". When you want to say that you want to go to the park with seojin, you'll say, 나는 서진이랑 공워네 가고 싶어". I think its the same order as in English for the case of location where you want to go or you go to emphasize on location. Please correct me if Im wrong.