PVC's can be caused by a multitude of things, some of the majors being: Hypertension, Stimulants (caffeine, exercise, alcohol, "diet" medications), certain medications, mitral valve prolapse, previous heart attack, electrolyte imblances...and a lot of times they are simply idopathic; in simple terms that means "it just happens and no clue what causes it."
PVC's can become serious if you have a bunch in a row.