What led to the defeat of the communist party in czechoslovakia?

I'd go with student and popular protests. The reason these succeeded was because in 1989, a lot of things were happening in Eastern Europe to lend momentum to the communist party's end in the country. The Berlin Wall had fallen and more protests were being waged in neighboring countries like Romania and Hungary against the communists, eventually dissolving the Warsaw Pact in the years ahead. The communist party in Czechoslovakia knew their time was up, so they gave up power in the face of the largely peaceful uprising that swelled across the country.