User blog comment:B214/Volume 22 spoilers/@comment-9595671-20160808174405

I think I just realized the reason behind why Rias traded Rossweisse to Issei. All of the pieces Rias traded with Issei were the members of her peerage that joined after she reincarnated Issei, while those stayed with her were all reincarnated before him and had been with her a long time.

Maybe Rias figured that everyone she added to her peerage after Issei were closer to him, and since Asia and Xenovia planned to be traded into his peerage early on, she must have figured Rossweisse was closer to him, especially after Volume 17's events, and decided to trade her into his peerage as well.