This is the article I linked to earlier. It's since been expanded somewhat:
http://www.app.com/story/sports/college/2015/02/11/seton-hall-basketball-jaren-sina/23231845/
Some highlights:
Sina did not want to air details, but the situation in the locker room had become untenable for him.
Internal strife has been a prime contributor to the downslide, as friction between the freshmen and upperclassmen grew over the past few weeks. After Saturday’s loss to Marquette, senior forward Brandon Mobley indicated some players were more concerned about their shot attempts than winning games. After the Georgetown loss, head coach Kevin Willard barred reporters from interviewing players for the first time this season.
One of the first highly-rated recruits to commit to Willard,
Sina had been a model citizen for the Hall in the classroom and on the court. He never had discplinary issues. Willard and others praised his work ethic and unselfishness. At times, Willard goaded him to be more selfish in hunting for his 3-point shot.
Generally speaking, Sina had a good relationship with Willard, who allowed him to call sets and considered him to be a coach on the court. That makes his departure, and the locker-room problems that led to it, all the more shocking.