As the nuclear charge is increasing, which holds the electrons more tightly to the nucleus, while the extra electron is also added to a 2p shell so it isn't any further from the nucleus and is at the same energy level. There is no additional electron shielding either, so the increased positive nuclear charge isn't counteracted and the electrons are pulled closer and held more tightly. To then remove the outer electron, much more energy is needed to overcome the electrostatic attraction, so the 1st ionisation energy increases.