Think he was butting heads with some players (Gasol) and management. They got off to a really bad start of his 2nd year there, then he benched Marc in the 4th quarter of game that ultimately lead to their 8th straight loss and he was gone.
Team wasn't good though, hey had moved on from ZBo, Allen, Vince, Mike Conley was hurt, they had just given all that money to Parsons, and the younger guys they took a chance on Mario Chalmers, Ben McLemore, Andrew Harrison, James Ennis, were all booty cheeks. But he was also trying to cut Marc out for guys like Jarrell Martin and Deyonta Davis, and neither of those guys were even NBA roster caliber guys.
Think he took the fall ultimately from a poorly constructed team, but had also alienated their best player, and Mike was out too. Only thing good that came out of that team was Dillon Brooks having a solid rookie year especially for a 2nd round pick, and JaMychal Green looking like he could be a solid part of their core for the next few years... (but then they traded him for Avery Bradley for whatever reason).
Those ****ty 2-3 years though led to Kleiman replacing Wallace, hiring Jenkins, drafting JJJ, Clarke, Ja, Bane, flipping Gasol for JV then for Adams.. All of which have been really positive moves for a quick turnaround.