Is it possible for an individual's TOEFL score to decrease between exams?
It is possible.

As an International student, I have taken TOEFL 4 times in one and a half year. Personally, I increased my score in every section at every exam. 

However, it does not suggest you will always improve your score. The reason is simple. Assume you do not spend time on learning English regularly between two exams. In fact, your score is a random variable with constant mean and variance. You may have got 28 in reading last time and 26 next time.

If you spend a lot of time on improving English skills, you may still get a lower score in writing and speaking sections. For those secitons you can apply for rescoring service.