This is because the #OH^-# ions from #NaOH# (#NaOH# #rarr# #Na^+# + #OH^_#) neutralise the #H^+# ions from #H_3PO_4# (Phosphoric Acid).
The equation for the dissociation of #H_3PO_4# is;
#H_3PO_4# #rightleftharpoons# #H_2PO_4# + #H^+#
The concentration of #H^+# ions on the right hand side of the equation therefore decreases, and the point of equilibrium shifts to the right, in order to oppose the decrease in concentration of #H^+# ions. The point of equilibrium shifts to the right, by more #H_3PO_4# dissociating, regenerating #H^+# ions, thus decreasing the concentration of Phosphoric Acid.
