Hux insists he can concentrate hard enough to withstand Kylo's mind-probing. Kylo sees this as a challenge and they proceed to experiment... except that he tries so hard that when he breaks through the mental wall, he sees too much. He sees a young Hux, victim of repeated abuse (up to the author what kind), and when he snaps out of the probing, he finds Hux on the floor, sobbing and feeling the flashbacks vividly.