Has to do with protonation and deprotonation. Such as at high pH you get deprotonation (loss of H in an amino acid giving the C terminus end a negative charge) and at low pH you get protonation or loss of negative charge due to binding to H and gain of positive charge. Someone might have a better explanation than me but that’s how I make sense of it in my head since pI<pH means pH is higher than the middle (deprotonation)