It is not always clear why a heart attack occurs and what you call a mini heart attack is probably what is known as a 'silent' heart attack, so called because the individual is not always aware that they are having, or have had a heart attack. The symptoms may be too mild for the real cause to become apparent.
However, the cause of the attack will not differ from that causing major heart attacks. The various heart problems can lead to a heart attack and the cardiologist will explain which of these conditions has been associated with, or the cause of the attack. Coronary artery disease is the most likely cause but the individual case will be carefully studied and advice given.