The greater the number of collisions the greater the pressure .
As the volume decreases the number of collisions increases.
Pressure is determined by the number and speed( momentum
( a condition for Boyle's law) then the molecules are moving at the same average velocity so the speed and the momentum of the collisions remains the same.
As the volume decreases the molecules are forced closer together increasing the number of collisions. These increased number of collisions causes an increase in the pressure.