What is the reason of happening …

1.The First World War ended with the defeat of Ottoman Turkey.
2.There were rumours that a harsh peace treaty was going to be imposed on the Ottoman emperor—the spiritual head of the Islamic world—the Khalifa.
3.To defend the Khalifa’s temporal powers, a Khilafat Committee was formed in Bombay in March 1919.
4.A young generation of Muslim leaders like the Ali brothers, Muhammad Ali and Shaukat Ali, began discussing with Mahatma Gandhi about the possibility of a united mass action on the
  issue.
5.Gandhij i saw this as .an opportunity to bring Muslims under the umbrella of a unified movement called ‘Khilafat and Non-cooperation Movement’.

The Khilafat Non-Cooperation Movement was launched by the Indian national Congress in 1920 under the leadership of Gandhi. Main causes for launching the Non-cooperation Movement were:

1. The treatment meted out by the British government to the Caliph of Turkey disturbed the Indian Muslims as he was seen as their religious head. As a result, they started the Khilafat Movement.
2. Gandhi saw this as an opportunity to achieve the much desired Hindu-Muslim unity. He advised the Khilafat Committee to adopt a policy of non-cooperation with the British government. The Khilafat leaders agreed and campaigned accordingly. Thus, the Khilafat Movement furthered the cause of the Non-cooperation Movement.
3. The Rowlatt Act of 1919 authorised the British government to arrest and imprison any person without trial and convict him in a court. Also, the Act implied severe restrictions on movements of individuals and suspension of the Right of Habeas Corpus. This was seen as a major breach of trust by Indians who were expecting the British to deliver on their promise of providing self-government. Gandhi appealed to the Viceroy to withhold his consent to the Act but his plea fell on deaf ears.
4. The Jalianwala Bagh massacre and the subsequent British reaction to it was a watershed event in the Indian freedom struggle. It led to a huge furor all over the country and hardened the Congress resolve to attain self government.
5. The Rowlatt satyagraha had been a widespread movement, it was still limited mostly to cities and towns. Gandhi now felt the need to launch a more broad-based movement in India which finally resulted in the launching of the Non Cooperation movement.