Question

why did Mahatma Gandhi decided to launch non cooperation movement

Answer 1

it is actually the reaction to opperessive policies of British rule in India against Rowlett act and Jallianwala Bagh Massacre.
during the movement indian Muslim who particited in khilafat movement gave their support to this MOVEMENT as well. it is sought to secure Swaraj in india, Gandiji promised Swaraj in indian if the non co-operation movent successfully implemented.
the other reason for movement is that gandhi lost faith in constitutional methods of British government and turned from cooperator to non-cooperator of British rule in India

Answer 2

Answer

Mahatma Gandhi decided to launch Non-Cooperation Movement  for self-governance and obtaining full independence as the Indian National Congress (INC) .