The "presumption of innocence" is a fundamental part of our legal system in the United States. It means that a court will not perceive you to be guilty unless you are proved to be so beyond a reasonable doubt. In this respect, it's up to the prosecution to present facts and evidence that prove you to be guilty.
Police must have a valid reason for pulling you over and accusing you of drunk driving. This valid reason might be as simple as you running a stop sign or not having your headlights turned on. Then, if the officer sees you're exhibiting the signs of inebriation, he or he she might perform a field sobriety check.
If you have ever been part of a motor vehicle accident, you know this has a way of "shaking you up."