Why did Gandhiji starts non-cooperation movement? …

Detailed answer:

1.After the suspension of the rowlatt satyagraha,Gandhiji felt the need to lauch a more broad based movement in India.This could be done only by bringing the hindus and muslims together.
2.The first world war had ended with the deafeat of the ottoman turkey and there were were rumors that harsh peace treaty was going to be imposed on the ottomon emperor-the spiritual head of the islamic world(khalifa).Gandhiji preferred to take up this issue in order to include muslims in the national struggle.The khilafat committee was setup to defend khalifa's temporal powers.
3.Many muslims such as the brothers,Muhammad Ali and Shaukat Ali started discussing with Gandhiji the possibilty of a mass action on the issue.
4.Gandhiji convinced other leaders in the calcutta congress session of September 1920 of the need to start the non cooperation movement in support of khilafat as well as for swaraj.Many within the Congress were reluctant to boycott the elections scheduled for novemeber 1920 and feared that the movement would lead to popular violence.However,a compromise was worked out and the non cooperation programme was adopted.

(i) After returning from Africa in 1915 Gandhiji launched some local satyagraha’s but he was looking for an opportunity to launch a national level movement against the Britishers.
(ii) The Non-Cooperation Movement was launched by Gandhiji to support the Khilafat
(iii) It was also launched against Rowlatt Act and the Jallianwala Bagh incident.
(iv) Gandhiji merged the Khilafat Movement with the Non-cooperation Movement to bring the Muslims under the umbrella of a unified national movement.