At page 27 of the white paper you can read about the consensus protocol. At page 31 there is a "Commitment" section. Basically, the nodes have X rounds (we didn't agree on the exact value of X yet) to reach consensus. If they don't, no reward is given to them, so it's in their interest to achieve consensus.
I'd argue that it is not in their interests to achieve consensus, as the reward is only transaction fees, which are small compared to the stake you can confiscate from nodes who behave badly.
Thereby, as a rational actor it is in your interest to make nodes behave badly by trying to disrupt the normal consensus, and then provide the proof, draining them of their stake.