the conservatives in Pennsylvania supported the change in the law that allowed for mail-in ballots.
Traditionally, older Pennsylvanians support conservative candidates. And with the state's population skewing older, Republicans in the state realized the thing that could dampen votes for their candidates was that the state's ancient, overly-tight restraints on what constituted a valid excuse for an absentee ballot meant that older Pennsylvanians who were shut in in nursing homes and couldn't find transportation to the polls increasingly wouldn't be able to vote for their candidates.
It was one of those bipartisan pieces of legislation that passed because Democrats believed the more of the population voted, the more would vote for them, and Republicans believed supporting the elderly's right to vote meant more Republican votes. In the end, they were probably both right.
Point is, the state Supreme Court rightfully saw through it when the people who'd supported the law in the first place called foul, but only in the third election the law was used, when their candidate wasn't the one who won.
... well, their candidate for President. An awful lot of their down-ticket candidates won. Which would be a weird thing to happen if the election had been rigged, wouldn't it?