After breaking out of the coffin/chambers where she was trapped Lady Madeline attacks Roderick for burying her when she was still alive. Launching herself at him out of anger and frustration, despite her still sick body.
The rest of the falling action is seen to be when Roderick falls to the floor dead in Madeline's embrace after the shock of his sister "returning from the dead". At this moment in the narrator takes the chance to flea the building out of shock and fear.
As the narrator suns away from the Ushers house from fear the building begins to collapse behind him. He gives a brief description of the waters of the tarn closing atop the splintered remains of the house and its inhabitants.