Many reasons. Many times the heart skips or beats irregularly in people who are otherwise normal. So most of the time we don't know. There are certain types of heart rhythm problems which are associated with actual abnormalities and disease in the heart. Examples are thyroid problems, anemia, heart muschle problems, and valve problems.
Palpitations from. Premature beats is most common & benign cause or other less benign arrhythmias.