Here are some reason. During excercise the muscles increase demand for blood and therefore the arterial system is dilated for increased blood flow. Additionally excercise in general increases the compliance of the vessels and as a result diastolic blood pressure is decreased.
Loss of body fluid. One can lose a significant amount of body fluid during a workout. The result can be a reduction of blood pressure; it could be a fall of one or both the systolic and diastolic blood pressure.