In a very brief and simple explanation....Jayden didn't like how often he was being told he is killing us defensively and Brad was going to keep telling him how poor he was doing defensively/how many scouting errors he had and one of them finally hit a tipping point.
So you land a highly recruited guard prized for his shooting ability, one Underwood described variously as an outstanding shooter, elite scorer, as good as any freshman guard in the nation, etc., somebody who fills a critical need on the team but then you have to plug him in as your PG after your other top freshman guard recruit bails out on you. And he plays well, having few turnovers and solid scoring from beyond the arc and able to often drive inside to score as well.
Rather than value the kid for filling the toughest role on the team while having no experience as a PG and boosting his confidence, you focus on his defensive weaknesses and criticize him to the point where he feels unappreciated, that he's being treated unfairly and wants out?
While coaches can't control everything about a player, their job is to evaluate players accurately, both physically and mentally, to get the best from them. Underwood seems to have failed with Epps, who will likely have a very good collegiate career. Of the five high school guards who have left during the past two seasons, Epps may be the one we regret the most.