If you go back to 1961, Borneo was divided into 4 separate entities: Brunei, Kalimantan, British Crown colony of Sarawak and British North Borneo (now Sabah). When UK withdrew from its SE Asia colonies, it wanted its colonies on Borneo to join the Federation of Malaya to establish Malaysia.
President Sukarno of Indonesia opposed the formation of Malaysia which he believed was a British puppet. He believed that Malaysia would help British control SE Asia and threaten the independence of Indonesia. Philippines also opposed it and made a claim for North Borneo for its links with the Philippines through the Sulu archipelago.
Indonesia and the Philippines agreed to accept the formation of Malaysia if a majority in North Borneo and Sarawak voted for it in a referendum organized by the United Nations. Did any of you vote in this referendum? The UN expected the referendum results to be released by 14th September 1963. Meanwhile, there's an anti-Malaysia movement in Sarawak, headed by Ahmad Zaidi.
Sarawak and North Borneo, expecting a pro-Malaysia formation result, declared independence on 31st Aug 1963, the 6th anniversary of Merdeka Day, before the referendum results were announced. On 14th Sep 1963, the referendum result was released to allow Malaysia to be formed on 16 Sep, 1963. Indonesia viewed this as a broken promise from British imperialism and the confrontation between Indonesia and Malaysia increased.
Sarawak's Merdeka day is really on 16 Sep, not 31 Aug. Merdeka, merdeka, merdeka!