The dicrotic notch on the aortic pressure curve is caused by
Ventricular systole begins with closure of mitral and tricuspid valve, due to increasing pressure inside the ventricle.
The first phase of ventricular systole is an isometric contraction. In this phase there is only increases in pressure inside the ventricle, but no ejection of blood happens. This may produce a small bump of aortic and pulmonary valve inside the aorta and pulmonary artery. A bump of the aortic valve may be seen in the aortic pressure curve.
After the isovolumetric contraction, the ejection phase starts with the opening of aortic and pulmonary valves. Aortic and pulmonary pressure builds and starts to decline towards the end of systole. When the pressure in the ventricles becomes less than the aortic and pulmonary artery, closure of aortic and pulmonary valves happens. Closure of aortic and pulmonary valves produces a small backflow of blood into ventricles producing diacritic notch.