Advice for making/keeping shredded chicken moist?
The first issue I see is that you are over cooking your chicken breast.  20 minutes in a pressure cooker is really over doing it.  Even a little over cooking dries chicken breast.  Chicken breast needs to be cooked more precisely.  A pressure cooker is not the correct tool for the job.  So you first need to correct your initial cooking.  You should get a good thermometer, ensure it is calibrated, and cook your chicken breast to an internal temperature of 160F (71C).  You can remove the chicken breast from the cooking surface when it is a couple of degrees below that.  Let it rest 10 to 15 minutes, and the heat will carry over and you will reach the final target temperature.  This will allow you to begin with moist chicken.