Why the increasing in the intracranial pressure causing bradycardia?

Complex: Look up Cushing Reflex. As blood pressure rises in an attempt to perfuse injured brain, baroreceptors in the aortic arch detect the high pressure and trigger a parasympathetic response via the vagus nerve... Voila! Bradycardia. There may also be mechanical stimulation of the parasympathetic response from the increased pressure in the brain.

Pressure on brainste: Increased intracranial pressure if high enough, will cause pressure on the brain stem, which controls heart rate.

Parasympathetic resp: Part Cushing Triad/reflex: Caused by the increased intracranial pressure (ICP) that initially increases heart rate and cardiac output to improve cerebral ischemia but then the aortic barorreceoptors detect it and trigger a parasympathetic response via the Vagus nerve (second phase of Cushing response). Direct distortion of the Vagus by elevated ICP can also trigger bradycardia directly.