Trump knew that mail-in votes would favor the Democrats, so he spread a conspiracy theory that mail-in voting was rampant with Democratic ballot fraud (it wasn't,) going so far as to attempt to defund the Postal Service to prevent mail-in voting altogether[0].

Of course, this meant Republicans avoided mail-in voting en masse, so when the (primarily Democratic) mail-in ballots came in after the initial numbers appeared to favor Trump, and the tide turned against him, the cries of fraud only became louder.

That's what Trump does, he poisons any well he can to harm his opponents, even if he has to drink from it afterwards. It's political because he made it political, the way he made masks and vaccination political because he thought COVID would distract from his narrative of a "roaring economy" and because he thought wearing a mask would make him look weak in front of the press, who he considered his enemy.

[0]https://www.brookings.edu/blog/fixgov/2020/08/24/why-trumps-...

That's both not true and trivially checkable.