Why is Bertie's hiding spot in Florence's closet revealed to Stilton?

When Stilton knocks at Florence's door, she hides Bertie in her closet. Bertie can hear Florence and Stilton's argument. Stilton accuses Florence of carrying on a constant affair with Bertie, after which Florence says that she intends to marry Bertie. This comment makes Bertie jerk in the closet and his hiding spot is revealed to Stilton when some boxes come crashing down onto his head.