For me it was all the fake polls that flooded the discourse in that last month. I changed my prediction about a week before the election in both Pennsylvania and Arizona Senate to favor the Republicans.
Conventional wisdom still had Kelly winning. But most people also thought Laxalt would win NV, and while a runoff in GA was considered more likely than not it was expected that Walker would lead the first round.
We also saw this with Hassan's race. And in the governor's races a lot of people believed that Whitmer was now in a dead heat with Dixon.