Why does ionisation energy to remove electrons from the same subshell increases?
The ionisation energies would be same if there was no shielding effect but it exists. The electrons due to mutual repulsion  prevent the outer electrons from the nuclear pull. Thus the force with which the nucleus attracts the electrons decreases as we move away from the nucleus both because of the increasing distance and also due to the increasing shielding effect of the inner electrons . Now in the same shell when one electron is removed the shielding effect decreases . The shielding effect due to the electrons of the same shell is the greatest. Thus the successive ionisation energies of any atom increases due to decreasing shielding effect.