Why did Brezhnev's 'social contract' break down?

In politics, a social contract is some kind of agreement, usually implicit, between governors and governed. It's a reciprocal agreement: those in power agree to provide something in return for a tacit acceptance from those who are governed. In the case of the Soviet Union under Brezhnev, the Communist government tacitly agreed to provide a certain level of social benefits and job security in return for a degree of compliance from the people. The USSR was a police state, and so opportunities for public complaints over government policy were virtually non-existent. But the government knew that the stability of the Soviet system depended upon a certain level of social peace. If people had a measure of economic security, then they'd be much less likely to resist.
There was, however, a fundamental flaw in Brezhnev's strategy. The Soviet economy was simply too inefficient and too resistant to change to deliver the economic benefits expected of it. The sought-for stability came at a cost: chronic stagnation. Industrial peace had been bought by an unsustainable policy of full employment, which led to Soviet industry being increasingly staffed by unskilled workers. The results were inevitable. Industrial output fell dramatically as productivity plummeted.
The process of decline accelerated further as already limited resources were increasingly devoted to defense spending rather than consumer goods. The inevitable shortages undermined the very foundations of Brezhnev's social contract. The black market flourished and corruption began eating away at the economic system from within.
As a devoted Communist, Brezhnev was never prepared to challenge the system to which he'd devoted his whole political life. His attempt at constructing a viable social contract must then be seen as a sticking plaster solution to cover up the myriad contradictions at the heart of that system. It was almost inevitable that he would fail. In due course, the unresolved tensions of Soviet economic planning led directly to the USSR's eventual collapse, which was to a large extent the culmination of a long period of stagnation begun under Brezhnev.