#3 Rumblings with the board
Part of the reason why Mourinho was appointed at United when there were misgivings within the board about his stability was that club needed a quick fix to steady the ship, and there is no one better at this than Mourinho.
United very well knew that Mourinho never lasts more than three seasons at a club, and brings problems as well as trophies, yet they wanted someone to re-instill the winning mentality as they waited patiently for his long term successor.
Zidane might seem an affable character, but he is a stern man on the inside who did not give in to Perez's demands when it was unreasonable. Whether it be selling favourite James Rodriguez, or benching Isco and Bale in favor of Asensio and Vazquez.
It was not a big deal as long as the trophies were coming, but as Madrid stuttered domestically last season, and as Zidane's transfer targets were vetoed, it was the final straw for the Frenchman who got tired of his president's antics, and decided to leave
While the owners at United do not interfere in footballing matters as much as Perez at Madrid, the blatant refusal of Woodward to back Mourinho in the market, and keeping the players Mou wanted to sell, as well as offering them new contracts, showed that he doesn't have faith in the manager.
Zidane walked away from Real Madrid when he felt he was being undermined, and would not hesitate to do the same at Old Trafford if the Glazers continue their recent strategy of prioritising commercial success over on field success.
Changing managers constantly without changing the flawed recruitment strategy will not yield success, even if a manager who successfully maintained egos at Real Madrid for three years is appointed.