For one the turnout for 2020 is a record number since... ever.
I suppose mail-in ballots with 0 signature verification had something to do with it.
Mail in ballots did, sure. So did an increase in early voting.
Remember, the GOP has been actively trying to suppress voting for decades.
There is a reason the last time you saw turnout numbers like this they were right after the Voting Rights Act. That provoked the backlash to find other ways to stop "the wrong kind of voters" from voting.
You also see big jumps in voting when people feel they fucked up by not voting last time. So after 2000, you see a 5% jump in 2004. Then in 2008 it goes up again because "we fucking need to get the GOP out, Bush ruined everything".
Voting ticks down again because Obama years are stable.
The media pushes really hard in 2016 for "they are both terrible, don't bother voting" so voting stays low and you get more wanker 3rd party voting.
The voting population reacts with "Holy shit, maybe voting matters" in response to all that and so things shoot up again.
Now, if the John Lewis Voting Rights bill gets passed, we are likely to see numbers stay in the 60s for a while, I bet.
The GOP is actively pushing at the state level to restrict voting further, though. (Separate from the states where they are passing laws saying that voting in Presidential elections doesn't count and the legislature can just overrule the vote and decide the winner.)