It was the decay of slavery. Bourgeois historiography puts the subjective causes first. They regard slavery, feudalism and especially capitalism as eternal categories. But the internal struggles of the leaders, the murders, the poisonings, all this symbolized the decay of the institution of slavery. Nevertheless, we don't want to deny the role of leadership.
Leadership is crucial when the objective situation is ripe.
But leadership is not a substitute for the class. All history attests to that. According to Marxist doctrine, no social system ever passes away without first fully exhausting its possibilities. The USSR had not exhausted its possibilities for growth. Its growth was aborted by a combination of internal corrosion and external pressures.