It's a myth that they were "all for Biden".
Wisconsin has a law that mail-in ballots can't be processed and tallied until election day, which was never an issue since prior to this election since there was a minimal amount of mail-in votes in Wisconsin before COVID (note that Democrats in Michigan wanted to change this law this year to avoid the scenario where ballot counting would last for days and raise doubt about the election, but the Republican-controlled state houses in Wisconsin voted it down).
Additionally, Trump encouraged his supporters to avoid voting by mail because it was "fraudulent" while Biden encouraged his supporters to vote by mail for public safety reasons, thus you had this lopsided effect where in-person votes skewed Trump and mail-in votes skewed towards Biden. In a state like Wisconsin where mail-in votes don't get processed until election day, Trump was always going to jump out to a huge lead. The jump you're seeing on that stupid chart is when a bunch of mail-in votes from Milwaukee County (i.e., black people in Milwaukee) were counted, and those naturally skewed heavily towards Biden. They were mail-in votes from Dem-heavy areas.
Compare this to Ohio, where mail-in ballots get processed BEFORE in-person votes did. On election night Biden jumped out to a huge lead on Trump because mail-in votes favored him, but as in-person votes were counted and as mail-in votes had already been counted Trump ultimately won out. That wasn't su ious at all and neither was Wisconsin. Each state has its own laws/process for counting votes, and the common theme is that in-person votes went for Trump and mail-in votes went for Biden.
This claim about election victory is disputed
Learn how voting by mail is fraudulent and insecure