Why did the inhabitants of Communist countries in Eastern Europe feel it was safe to rebel in 1989?
The Soviet Union maintained its power through coercion and terror, in the late 1980s it lost the power and will to continue, and disintegrated.
From the Bolshevik Revolution in November 1917 onwards, the USSR maintained its rule through military power and ruthless security agencies.
The Cheka/OGPU/NKVD/KGB was an external intelligence agency but also ran internal security, a border guard force, staffed the Gulag camps and was a paramilitary force to guard key sites. They also did most of the dirty work in running the Great Purges.
The Soviet military was charged with the external defence of the USSR, but was also used to support internal security against various rebels and partisans after the Civil War and the Second World War.
The Party also kept tight control over both (and both watched the Party).
In the 1980s, Gorbachev became the first Soviet leader without blood on his hands. All of Stalin's successors had been involved in the machinery of repression -- often in a 'hands on' role, except for Gorbachev. Moreover, the USSR could no longer control all aspects of internal communication and found their hands tied by international agreements to respect human rights.
The USSR had -- with some reason -- always placed a strong focus on a very strong military; and never really stood down after 1945. This became a compulsion, but also the USSR was committed to someday 'spreading' the Revolution and the massive military threatened the neighbours (particularly the US and Western Europe).
As the Soviet build-up continued in the 1970s,it provoked a counter-build up in the West... but the new US/European weapons were of a quality the USSR could never hope to match without a massive reinvestment. In the 1980s it was clear the USSR was going broke.
Gorbachev had to relax the Soviet grip on their minorities and on their allies in Central Europe; and that was enough. In Poland, Czechoslovakia, Hungary and East Germany, people sensed the grip was relaxing and moved to free themselves. Soon enough, other peoples inside the USSR followed, and by 1991 it was dismantled.