Existing state law meant ballots had to be received by 8 p.m. on Election Day in order to be counted. The Democratic Party filed a lawsuit to extend that deadline and the Pennsylvania state Supreme Court (not SCOTUS) made a highly controversial ruling that extended the deadline to the following Friday. This extension would have helped Biden (given his party filed the lawsuit to force the change), and given they barely won the state (Biden had 50.01%), there is a good chance it affected the outcome.