It pays to learn the nuances and quirks about how the test is scored. For example, don't believe the old cliche if you don't know the answer to a question you should just go with "c". As it turns out, you would do better to not answer at all if you're not sure, as a non-answer counts less against you than a wrong one.
The atmosphere of the room can be a distracting factor for some students. Many times, the SAT is given at a central location other than the student's school. The change of venue can be difficult to adapt to for a variety of reasons, from stuffy rooms to uncomfortable desks and the like. Plus, the rigidity of the environment---for many it's the first time their bathroom breaks have been strictly regulated, for example---can be intimidating for some.
The more involved a student is in extracurricular activities, the less time he has to comfortably prepare for the SAT. Granted, some students make time and sacrifice sleep, but that creates another problem. Doing more activities or sleeping less creates an increased risk that the student will not be as focused. It doesn't guarantee a lower score, but the odds of it happening may increase slightly.
The SAT is different than traditional tests in that the student can only get a general idea of what he will face, as opposed to, say, a geography exam where he can can feasibly prepare for virtually all of the specific material. However, practice tests and a simulated test room environment experience can go a long way toward boosting a score. If a student goes in with minimal preparation, it is often reflected in his diminished score.