He said that the decision is taken to let another person take charge of the country.
Reportedly, it is expected that chairman of the Communist Party of Nepal (Unified Marxist-Leninist), Khadga Prasad Sharma Oli may become the next Prime Minister.
Posted by Anup Baral October 10th, 2015 :

Senior leader of the party Sher Bahadur Deuba proposed Koirala as the PM candidate and party Vice-President Ram Chandra Paudel seconded him.
Before filing his nomination, Koirala had called on President Ram Baran Yadav and submitted his resignation from the incumbent PM position.