Good question! We know the who, what, when and where. Now only the why remains.
Monday, the president needs to request the house convene so to make his State of the Union address to the nation at the set time. If she refuses, then activate his legislative authority under Art 2 Sec 3 of the US Constitution.
I would pay good money to watch the Sergeant at Arms round up the wayward congress critters to attend to insure a quorum, as is enabled by the Constitution and law.