In the book, So B. It, why did Bernie close the door for the first time ever?

Bernie was angry at Heidi for buying the bus ticket and going to play the slot machines, when she believed that Heidi had gone to the library.... but even more importantly, she was angry at Heidi for lying to her. In her anger, she stomped out of the apartment and slammed the adjoining door behind her. This is the only time Heidi remembered seeing the door closed.