How does a spacecraft change course?
Basically, all spacecraft go off course because our understanding of the universe is not complete or detailed enough to perfectly plan a spacecraft's flight. Also, there are many factors that influence space flight, and we don't have the ability to account for all of them in space at once.
As DS1 goes through its journey, the gravity from the Sun, the Earth, Mars and Jupiter will all be pulling on it, as will gravity from asteroids. The amount of gravity from each of these things will change constantly throughout the journey as DS1 gets farther from one object and closer to another. We do not have the mathematical ability to model all of those gravity pulls correctly so that they change as DS1 moves. In addition, solar wind will push on DS1, affecting its course. The push of solar wind will also change over time. A change in the ship's speed could also change the trajectory itself, or else delay the ship so that it can't make its rendezvous with an object in space.
Unlike most spacecraft, which are initially sent into space with a huge initial push and then coast for most of their journey, DS1 will be firing its thrusters close to continuously. Any small error in these firings could put DS1 off course and small errors happen often.
Whenever any event happens so that a spacecraft gets off course, a course correction must be done. DS1 and AutoNav just routinely do course checks and corrections.
What is a course correction?
What is AutoNav?
How do we know a spacecraft's location?
How does NASA run space missions?
What would happen if DS1 collided with an object in space?
How does DS1 do a course correction?
When does DS1 do a course correction?
How is NASA overseeing the DS1 mission?
What is thrust?