Why did Mamata Banerjee leave Congress in the 1990s and form TMC?

Mamata Banerjee left Congress in the late 1990s. Mamata increasingly felt that the Congress high command was deliberately trying to stop her anti-Left movement in West Bengal, reported NDTV. After being expelled for announcing plans to contest elections under a new symbol, she founded TMC. Years later, Congress has now asked Mamata to return as TMC is facing politcal crisis.