Question

why did the congress decide to launch the civil disobedience movement?

Answer 1

The British Government in India had deprived the Indian people of their freedom. It had based itself on the exploitation of masses,
The British Government had ruined India economically, politically culturally and spiritually.
The people of India believed it to be a crime against man and god to submit any longer to a rule that has caused irreparable harm to Indian economy and its social fabric.
The Congress, therefore, declared the attainment of complete independence as its aim in the Congress Session held at Lahore and decided to launch Civil Disobedience Movement under the leadership of Gandhiji.

Answer 2

Civil Disobedience movement(1930-1934) had 2 phases. One before Gandhi-Irwin Pact and second after 2nd RTC. Because of signing of Gandhi Irwin pact, Gandhi agreed to discontinue civil disobedient movement. After that gandhi went on to attend 2nd RTC but returned disappointed. By this time viceroy of India was changed and He was arrested soon after he reached India along with other major leaders. The communal awards were announced. He was shocked by the reality of Dalits and their presentation by Dr. Ambedkar. So he decided to quit the active politics & work for the upliftment of Harijans. He launched Individual civil disobedience on August 1, 1933. Civil Disobedience movement continued till April 1934 when Gandhi formally withdrawn it. 

RTC refers to Round Table Conference.

:)Hope this helps!!!