How did Lindsey and Hatch's car end up in the ravine in the novel, Hideaway?

The couple's car ended up at the bottom of the ravine when they lost control on the highway above. While trying to avoid a truck that had crossed the lanes on an angle in front of them, the front bumper clipped the semi, and the car slammed into the guardrail until it gave way and sent them tumbling into the ravine.