Trump discouraged and rallied against mail-in voting for months; so most of his voters voted in person. Meanwhile Most Democrats voted by mail because they wanted to limit social exposure during a pandemic. Mail-in voting takes longer to count, so the concept of the election starting out looking favorable for Trump, then switching over to Biden is not that far-fetched. It was actually being predicted for quite some time before the votes were even cast.

My favorite thing about these sorts of images is that they misrepresent the very graph they claim shows voter fraud in Wisconsin. Look closely:

Both Trump and Biden saw a sudden influx of votes at the same time--not Biden alone, as the image suggests--and the race tightened to a dead heat rather than Biden suddenly running away wiht things. But because the blue is overlaid on top of the red, you don't see the red jump. Incidentally, this is when Milwaukee reported a large batch of the results, an area where Biden could be expected to outperform Trump. That Biden received most of the votes from that report, but not all of them, is unsurprising.