Pressure Decrease in Fluid Flow

In a laboratory, a scientist is studying the flow of a viscous fluid through a horizontal pipe. The pipe has a diameter of 4 cm at one end and narrows to 2 cm at the other end. As the fluid flows from the wider section to the narrower section of the pipe, the scientist observes a decrease in pressure. According to the principle of fluid dynamics, what is primarily responsible for this observed decrease in pressure?