D'Antoni is clearly the biggest reason Dwight could be leaving. But, the Lakers keeping Pau for too long is number 2.
They should've looked to trade him for younger talent when he had more value. It's something they should still look to do, but I doubt he returns enough young talent anymore. I called it before last season began. They needed to deal him as soon as they traded for Dwight Howard.
Steve Nash is number 3. They should've used those draft picks to go after young talent instead of trading for a 38(at the time) year old PG.
The promise of 2014 cap space is nice, but it's no guarantee. I doubt Dwight feels good about tying his future to that alone.