The Starks have ruled Winterfell for 8000 years, which means that the castle contains thousands of corpses who could potentially rise at the command of the Night King.
Fans online also noted that there's a scene in George R.R. Martin's A Game of Thrones which Jon dreams of the Stark dead emerging from their tombs: "Last night he had dreamt the Winterfell dream again. He was wandering the empty castle, searching for his father, descending into the crypts. Only this time the dream had gone further than before. In the dark, he’d heard the scrape of stone on stone. When he turned he saw that the vaults were opening, one after the other. As the dead kings came stumbling from their cold black graves, Jon had woken in pitch-dark, his heart hammering."
So, while we don't know anything for sure, we think something bad may be going down in the crypts.