What was the big lie in regards to the 2020 US presidential election?
The big lie in regards to the 2020 US presidential election was the false claim made by former President Donald Trump and his allies that the election was fraudulent and stolen from him. This claim was widely debunked by election officials, experts on both sides of the political aisle, and even President Trump's own Attorney General. The false claims of widespread voter fraud ultimately led to numerous lawsuits being filed against the election results, all of which were ultimately dismissed or denied.
What do you think caused the claim to spread so quickly
The big lie about the 2020 US presidential election spread quickly because of a number of factors. 
Firstly, the modern age of information and the prevalence of social media meant that false claims could spread rapidly and easily. 
Secondly, political polarization and the echo chambers it creates meant that people were predisposed to believe the claims.
Thirdly, the profit incentives of the news media meant that sensational headlines and clickbait articles were prioritized over accurate reporting, which furthered the reach of false claims.