The reason is that Pennsylvania's PA vote was finally decided. Biden won Pennsylvania's 20 electoral votes, more than half of the total (270). If the result is upheld, Biden would have the last laugh in the 2020 election and become the 46th President of the United States.
The map clearly shows that PA has been taken by Biden, from red to blue, which means that even if Trump wins all of the remaining undecided states, he won't have the votes he needs to win, and Biden will become the next U.S. President if nothing goes wrong.
Trump himself tweeted, "The legal proceedings are just beginning!" With the Democrats in doubt in several key states, Mr Trump and his team are ready to embark on a lengthy legal battle to overturn the outcome.