It's never explicitly stated and Jun himself is unreliable as he thinks its about Yohan just messing with Jun. That being said, in consideration of Yohan's character and what we learn about him later in the novel, he probably just felt really humiliated and played by Jun in the moment with his family and acted out. Afterwards Yohan probably thought he'd teach Jun that he needs Yohan to survive school and not play with Yohan's feelings again. Which we know backfired because Jun has crazy pride, and when Yohan couldn't take the cold war, since it dragged on longer than he thought it would and Yohan himself is obsessed with Jun, he started begging Jun instead to talk again.
okay tyysm!!!
i forgot why but does anyone remember why yohan neglected jun for a while and treated him bad then suddenly got over it and now hes friends with him again?