Lots of possible reasons for this over-the-top speech.
#1: More revelations about emails, Clinton Foundation, etc; for instance the fact that 50% of all dignitaries Clinton had meetings with during her tenure as SecState were donors to her fund. Something that neither she nor the MSM would like to discuss.
#2: Health issues, true or not, the fact that the questions about her health have started to spread not only through the most distant reaches on the internet, but now also in MSM makes it imperative for her to either answer in a trustworthy fashion (stop laughing, you in the back), or direct the discourse elsewhere. (Even if her problems are only mild back-, hip-, or knee-pain the optics compared to Trump would be/is very bad.)
#3: Trump’s outreach to the African-American community was something the Democrats probably never had counted on. Problem is that not only did Trump make a very good case for himself, but even if it is only a minority of the AA who will leave the Democrat plantation, I think it puts a spanner in the Democrat get-out-the-vote effort. Previous elections the Democrats could count on every voter in the inner-cities they got from the couch to the voting booth was one of their votes. Trump’s gambit threatens that whole strategy.
I think that is three reasons why this speech had to come now, and why it was so way over the top. Add to these issues the fact that maybe the Democrat internal polling does not look as good as the MSM-polls, and you have a campaign in dire straits.