Why did Gandhiji decide to withdraw the Non - Cooperation Movement?

Explanation:

Mahatma Gandhi led the non-cooperation movement that started on 5 September 1920 by the Indian National Congress.

The party began the Non-Cooperation Policy in September 1920, at the Congress meeting in Calcutta.

Gandhiji wrote a manifesto in which he wrote the ideology of the non-violent non-cooperation movement in March 1920.

Final Answer:

At Chauri Chaura in Gorakhpur, a peaceful mob turned violent and clashed with the police, that resulted in the deaths of several policemen.

Mahatma Gandhi, realised that the movement has  becom violent and that the satyagrahis required some proper training before being ready for the collective struggle.