A few things that stick out to me as possibilities:

• Her awkward personality is a slow burn grate on the populace. Probably no single moment here, just a built up of events and speeches that communicate no charisma.
• It’s been reported that both her primary campaign in 2020 and her VP outfit have been marred by mismanagement, in-fighting, unclear chains of command. Likely her presidential campaign is more of the same. Besides that generally being a hindrance, if disgruntled staff start talking to the press that’s going to create the image of a sinking ship which will put off voters.
• The genocide in Gaza starts getting much, much uglier. I don’t think the Palestinian issue is front and center for most voters, but if the atrocities get more blatant and Harris keeps defending Israel, it could sour enough voters to swing some states.
• More “Pell-grant recipients” policy platforms. This would likely tie in with the first issue; voters don’t like these overly wrought, means-tested programs and if everything out of her is like that, it’s going to feed the sense of her being unlikable. Voters read triangulation as shifty.
• COVID makes a roaring comeback. Cases/hospitalizations/deaths spiking close to the election would undermine the spin that Democrats “beat” COVID, would drive a lot of liberal voters to stay home (but not conservatives) and the associated economic hit would be placed on Harris’ neck.
• Speaking of which: it’s the economy, stupid. People are already feeling like the economy is going the wrong direction with inflation of essentials, home and tuition prices through the roof, and interest rates up. Things get any worse, that will disillusion voters and as dumb as it is, there’s a not-insignificant chunk of the electorate in play that see Republicans as better on the economy.