Here is the tldr as much as I can remember anyway. Democrat senators knew that Trump's defense was really strong yesterday and they had no chance of conviction. Due to this, they voted to allow witnesses and extended the trial another day. Their plan was to have hearsay witnesses so they could effectively lie with no reprocussion. Lindsay Graham voted with the Dems to allow this in order to use this trial to raise up a different candidate. All that said, they opened a can of worms with this decision as Miller prepared a list of 301 witnesses. On the list were Nancy Pelosi, Kamala Harris, Chuck Schumer, DC Mayor Bowser, etc. Trump was going to use witnesses on his team to explain that he ordered the national guard the night before on January 5th, but was denied by said Democrats which proves innocence and puts them in the hot seat. They would get 2 hours of debate per witness causing this trial to continue another 604 hours for Trump's witnesses alone. Dems panicked hence the sudden decision to deny witnesses last minute.