I will give you two theories on this. One is that the whole thing is in the narrator’s head and the comments about how she seemed to be alive made him imagine her coming out of the tomb. The other option is that she was just so angry about how she was buried alive that she mustered up the strength to push her way out. After all, the lid was not tight because they are the ones who screwed in the lid.
To say that Roderick is not in his right mind is an understatement. The narrator really does not seem to know what he walked into when he went to that house. He hasn’t seen Roderick for ages. They were childhood friends. He came because Roderick asked him to, and he found himself stepping into a nightmare.
The doctor did not know what either Roderick or Madeline were suffering from; according to the narrator he “wore a mingled expression of low cunning and perplexity.” When Madeline died, Roderick’s reaction was odd to say the least. He wanted his sister entombed beneath the house. There, the narrator says she seems to look alive still.
The disease which had thus entombed the lady in the maturity of youth, had left, as usual in all maladies of a strictly cataleptical character, the mockery of a faint blush upon the bosom and the face, and that suspiciously lingering smile upon the lip which is so terrible in death.
This is one of the hints that foreshadows the fact that she is alive, in addition to the fact that she is being entombed there and not somewhere else. She almost seems healthy. You can imagine that she might have pulled together enough anger and resentment to pull herself out of the tomb and kill her brother. Roderick seemed to think it was possible.
Not hear it? — yes, I hear it, and have heard it. Long — long — long — many minutes, many hours, many days, have I heard it — yet I dared not — oh, pity me, miserable wretch that I am! — I dared not — I dared not speak! We have put her living in the tomb!
Maybe it is the twin bond, and Roderick knew that she was still coming. The whole thing is creepy, and maybe the creepy house and Roderick’s bizarre behavior made the whole story a figment of the narrator’s imagination. It could have been a nightmare.