Question: why was exo cbx formed?
Answer: EXO revealed why Chen, Baekhyun, and Xiumin were picked to be in EXO-CBX on Knowing Brothers. Apparently, Chen, Baekhyun, and Xiumin had the same gym trainer. That person suggested that these three should form a subunit, and CBX proposed the idea themselves to SM. This idea was accepted, and EXO-CBX was formed.