From your article, they are betting on Trump’s success in preventing people from voting:
Basically, unlike national polling, they are taking into account Trump’s corruption succeeding as it always has.Jennings said the reason for this is difficult to pinpoint but that prediction markets "can price in factors other forecasting methods can't, such as the potential impact of the pandemic on voter turnout and fears related to the logistics of actually submitting and counting voters’ ballots."
Oh and nothing to do with convention...
Edit: My avatar is there, because I agree with the betting... the only chance Trump has, is to impede or fuck with voting. Judging by his supporters still cheering him on for attacking American democracy, the odds of him succeeding are indeed increasing.