In order to answer that one has to explain what 'колея' is first. And that's a groove left in a country road by a wagon wheel (or a car wheel). So when your wagon runs along this groove, it's basically on its usual path, not swaying left or right from it. But when something pushes you from the side, your wagon is off the groove and you don't have smooth familiar path anymore. You are in for some unpredictable, and potentially unpleasant things, you're in for a stress reaction.