Bruce, as a builder i am constantly checking freeze depths and it changes with the type of foundation, and if there is in the example of a mobile home, skirting around the piers / foundation.
In your area, the normal foundation freeze depth is 36 inches, which you are at, but the table i see with your situation, pier with no skirting, is 48 inches, i believe this is why you have the frost heave.
here is a link to the PA frost tables, pdf file page # 4