will Contests, Who May Contest
Any person or entity that will receive more of the testator's estate if a will is not accepted for probate may properly contest that will.

For instance, this could be someone who wants to have another one of the testator's Wills to be followed, which may be an earlier or later will than the one that has been offered for probate.  It could also be a family member who wishes to have the will declared invalid so that the decedent's property is distributed according to Pennsylvania's intestacy laws.

See: Will Contests, Generally; Fraud, Generally; Sound Mind, Generally; Improper Execution, Generally; Witnesses, Advantages