The shift to the left that the new "General Line" represented was brought on by a combination of domestic and external factors.
Although the party leaders appeared generally satisfied with the accomplishments of the First Five-Year Plan, they--Mao and his fellow radicals in particular--believed that more could be achieved in the Second Five-Year Plan (1958-62)
The Great Leap Forward was an economic failure.
shortage of food
shortages of raw materials for industry; overproduction of poor-quality goods; deterioration of industrial plants through mismanagement; and
exhaustion and demoralization of the peasantry and of the intellectuals, not to mention the party and government cadres at all levels.