User blog comment:B214/Question and Answers 6/@comment-185.112.105.215-20170331204546/@comment-10330822-20170413071835

Crom probably joined Rias's team because of his connection to Gasper through Balor, as well as the fact he wanted to fight against people like Issei, Dulio, and Vali. I don't think anyone in Rias's team interests him, so its the perfect team to join.