Why does sinus tachycardia typically develop?

Tachycardia is a heart rate of more than 100 beats per minute. Sinus tachycardia is the proper physiologic response to both physical and emotional stressors. With sinus tachycardia, the heart’s natural pacemaker in the upper part of the right atrium sends out electrical impulses faster than 100 beats per minute. Everyone with a normal heart will experience periods of sinus tachycardia. Some common examples include:
•    During exercise
•    Dehydration, or any cause of low blood pressure
•    Infection, especially with fever
•    Anxiety
•    An overactive thyroid (hyperthyroidism)
•    Side effect of medication