The answer would be salt hydrolysis.
Salt that was made from the combination of weak and strong acid/base could be hydrolyzed and forming another substance. The hydrolysis process will separate the H2O into H+ and OH- and the weak part of the salt will take one of the ions(weak base will take OH-, weak acid will take H+). The result would be increased H+( or OH- ion, leading to change in pH of the solution.