What causes the cardiac ventricular pressure change during the isovolumetric part of the cardiac cycle ?
Boyle's law doesn't apply to all fluids: only gasses, not liquids. Liquids like blood are (mostly) incompressible, so their volume does not change substantially when you add pressure. 

During isovolumetric contraction, the ventricular walls are pushing in on the blood contained in the ventricle, causing the increase in pressure, but there is not yet sufficient pressure to force the blood out the pulmonary/aortic valve.

Source:

Hillegass, E. (2016). Essentials of cardiopulmonary physical therapy. Elsevier Health Sciences.