If you are overweight, then this can cause high blood pressure. This occurs partly as the body now needs to work harder in order to move blood around your body and that in turn can cause your heart to beat faster and your blood pressure to rocket.
Likewise, a range of other issues can make high blood pressure likely to correlate with obesity. For instance, obese individuals are less likely to be physically fit, which in turn means that the heart needs to work even harder still. Likewise, being overweight can cause insulin insensitivity, which can in turn increase the amount of glucose in the blood, thereby increasing blood pressure.
Finally, they are more likely to eat a diet rich in sugar.