It was the BLM stuff. I clearly remember that by Mid May he was still within fighting distance of Biden and the election was considered lean Biden, but still completely winnable for Trump despite the Covid pandemic raging and the economy in shambles. He was down in the rcp average by <5% and had a lead in the betting odds. However, after the race riots and the end of June he was down double digits in the polling aggregate, and most people had Biden as a clear favorite (likely D). He managed to recover a good chunk of it by August, but the momentum stalled out after that and it clearly wasn't enough to win.
There was a second wave of Covid in June 2020 that could easily be blamed on Trump though because he was minimising the pandemic's impact and encouraging states to abandon restrictions quickly, so it was easy for him to come across as an out of touch fool. For much of the campaign period the US also had a lot more Covid going around than Europe (they didn't have a summer wave), but Trump didn't even look like he was trying to deal with it.
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