Being athletic. The reference range for creatine kinase is set based on the assumption that you do not exercise. In my series of medical students, all the men and the majority of the women who lead even moderately fitness-focused lifestyles have high creatine kinase -- often very high. If you're healthy and are working your muscles, be proud of yourself and keep doing what you're doing.
Muscle issues/MI. The main possibility to be concerned about is myocardial infarction. The ck-mb fraction would be particularly elevated here. This would fit with a total picture, like chest pain, etc. Other reasons for total creatine kinase elevation include im (intramuscular) injection, heavy exercise, and skeletal muscle trauma.