It was an unscheduled event. Biden decided he wanted to make a statement to the press to clarify some of his positions. He set up a time and place for the statement.
While he was waiting for people to set up the podium and clear an area and get things organized, Biden got out and was chatting with some of his local organizers. Some members of the press captured his chat. Biden wasn't talking to the press at that point.
When things got set up, people were getting the press moved into position for the statement. Press members can be seen turning off their cameras and moving toward the front of the podium.
While the press is getting into position, Biden is finishing up his chat with the organizers and supporters. Biden then walks to the podium and gives his statement to the press.