The Communist party was due to draft a new programme in November 1991. Parliament had adopted an "anti-crisis plan" to accelerate economic reform. The 12 Soviet republics that remained after the Baltics left had accepted the text of a new treaty that would give them more political and economic autonomy while leaving defence and foreign affairs to the Soviet government. The treaty was to be signed on 20 August.
"Here I made a mistake. I went on holiday. I probably could have done without 10 days of vacation … I was all ready to fly to Moscow to sign the treaty," he said. But on 18 August a group of people arrived uninvited. I picked up the phone to ask what kind of people they were and who had sent them, but there was no line. The phone had been cut off."
https://www.theguardian.com/world/2011/aug/16/gorbachev-guardian-interview
Was it because the Soviet republics lost faith in the governement? Although he said the republics already accepted the text of the treaty.
Well, first of all - the "anti-crisis plan to accelerate economic reform" and the "new treaty that would give Soviet republics more political and economic autonomy" are completely different things.
Measures to implement the anti-crisis plan included price liberalization, the denationalization of the economy, and the strengthening of the ruble based on anti-inflationary policies. (At the same time, the program also included things like the revival of spirituality, providing wide opportunities for young people, and the provision of social guarantees to the population on a legislative basis.)
The New Union Treaty was a draft treaty that would have replaced the 1922 Treaty on the Creation of the USSR and thus would have replaced the Soviet Union with a new entity. (Despite the fact that the treaty was officially called the “Treaty on the Union of Sovereign States”, the state it established was called the “Union of Soviet Sovereign Republics”)
And secondly, the treaty was to be signed on 20 August, but not by Gorbachev. On August 20, the new union treaty was to be signed by the RSFSR, the Kazakh SSR and the Uzbek SSR, and after some time by representatives of other republics, for which the following order was supposed: September 3 - the Belarusian SSR, September 17 - the Azerbaijan SSR and the Tajik SSR, 1 October - the Turkmen SSR and the Kirghiz SSR, and also supposedly in October the Ukrainian SSR. However, the State Committee on the State of Emergency prevented the signing during the events of August 19-21, 1991.
On August 23, Yeltsin signed a decree suspending the activities of the Communist Party of the RSFSR, and on November 6, terminating the activities of the CPSU. The defeat of the State Committee on the State of Emergency led to the collapse of the central authority of the USSR and the re-subordination of power structures to republican leaders. Within a month after the coup, authorities of almost all union republics declared independence one after another.
Ukrainian independence referendum held on December 1, 1991, at which supporters of independence won even in Crimea, made, according to some politicians, in particular Yeltsin, preservation of the USSR in any form impossible. On December 8, 1991, the heads of three of the four founding republics of the USSR - Belarus, Russia, and Ukraine, stated that the USSR was ceasing to exist, announced the impossibility of forming the Union of Sovereign States, and signed the Agreement on the Creation of the Commonwealth of Independent States.
Source: Drafting of the Union treaty in September-December 1991: from the dismantling of statehood to the USSR's collapse, Lukashin A.V., 2018