I remember that Kugo Ginjo used to be a Substitute Shinigami, but he got kicked out and lost his shinigami status. Why did this happen?
It is not that Kūgo Ginjō got kicked out. He left on his own accord after feeling betrayed by the shinigami.
Some time ago, Kūgo became the first Substitute Shinigami and in recognition of that was given a Substitute Shinigami badge by Captain Jūshirō Ukitake. How he gained Shinigami powers is unknown, but according to Captain Suì-Fēng, he murdered many Shinigami in order to steal their power. At some point, Kūgo found out that his badge was a surveillance device used by Soul Society to observe him and limit his power. After that discovery, Kūgo thought he had been betrayed and disappeared without a trace. - Bleach wiki