What can cause a rapid heart beat?

Rapid heart rate. Tachycardia can be physiological or pathological. Exercise, caffiene can increase heart rate. Meanwhile diseases of the heart, like atrial fibrillation, supraventricular tachycardia, congestive heart failure, ventricular tachycardia can cause heart to speed up.
Many things. Dehydration causes low blood pressure. The heart beats faster to compensate. Normal exertion should increase heart rate. Thyroid disease may cause it. And there are many abnormalities of the hearts electrical system that may cause a rapid heart rate.