I think the final straw that finished Hillary Clinton’s campaign was the revelation about Huma Abedin’s husband Anthony Weiner being heavily involved in the private e-mail server scandal. Given Weiner’s very sordid past even back in 2016, it couldn’t have been worse news for the Clinton campaign.
The FBI did not want to release the scoop on Weiners computer contents but they were forced to do by leaks from NYPD. As fast as they could they then denied that there was anything criminal there. They screwed up.