A group of five army majors, mostly Igbo, started plotting the coup in August 1965 because they believed many Nigerian leaders were stealing public funds, an act which was detrimental to the extremely fragile new state. The coup began after mutinous Nigerian soldiers in the north killed 22 people, including the Prime Minister Abubakar Balewa, senior politicians, and army officers. The coup plotters attacked three cities, Lagos, Kaduna, and Ibadan, while simultaneously blockading two rivers; their coup lasted two days before it was subdued. The coup led to the collapse of the First Republic of Nigeria.
Two days after the start of the coup, acting President Nwafor Orizu made a broadcast announcing that the cabinet was voluntarily transferring power to the armed forces, specifically General Johnson Aguiyi-Ironsi, also an Igbo. Ironsi then suspended the constitution and started the Supreme Military Council in Lagos. The majority of the coup conspirators were Igbo, and the January 1966 coup was one of the main causes of the counter-coup that occurred six months later in July 1966.
Sources: http://www.bbc.com/news/world-africa-35312370; http://allafrica.com/stories/201601150403.html; https://en.wikipedia.org/wiki/1966_Nigerian_coup_d%27%C3%A9tat