What are the causes of rapid heart beat on exertion?

May be normal respon. After exercise the heart rate will go up, this is a physiological response. And it should come base line in 10-15min of rest . If in case the heart rate continues to be high more than 100, then you should seek medical help.
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 beats.