Assuming there is no fraud, perhaps they're concerned that Trump can pull off a million to one shot and rip victory out of the jaws of defeat? I mean, it's not exactly unprecedented.
Or more realistically, they want to wholly crush Trumps populist movement before it can become powerful enough to threaten their power? That's what I can come up with if they didn't just cheat and are trying to desperately cover their tracks.
Even if he wins, Biden is looking to be just as devisive as Trump was back in 2016 except on the opposite end of the political spectrum. I think having a bunch of angry rednecks that handle all the agriculture not consider you a duly elected president is way more dangerous than a bunch of gender studies majors and journalists thinking the same.