Why is the movie titled "Skyfall"?

Skyfall Lodge used to be James Bond's home:

... the Bond family resided at Skyfall Lodge, though James Bond had not visited his ancestral home since his parents' death. Its only resident ever since has been gamekeeper Kincade.

After they died, Bond's parents were buried in the chapel on the grounds.

In an interview, Purvis and Wade, authors of the film, claim the title "fell from the sky":

Neal Purvis: “Unfortunately Rob came up with the title. Not as a title, he came up with the name of the house.”

Robert Wade: “We were looking for a name for this house…”

Purvis: “Like Manderley in (Daphne du Maurier’s) Rebecca.”

Wade: “Exactly. It’s evocative, and you don’t really know what it means. And it was two o’clock in the morning, and the script had to go off, and I typed ‘Skyfall’. So it sort of fell out of the sky, actually. And I never thought that that would end up as the title of the movie. But it obviously struck a chord. But if we’d thought the name of the house would be the name of the film, and the name of the song…”

Purvis: “We’d have talked about it a little bit longer.”

Wade: “And maybe have done a three-syllable house name. Like Manderley – that’s pretty good, actually.”

Purvis: “Yeah. It’s the first two syllable Bond title, actually. Some people find that interesting (laughs).”

Ian Fleming, creator of the James Bond character, never named the Bond ancestral home, but in You Only Live Twice, he did write that Bond's parents died in a mountain climbing accident, so interestingly, they fell from the sky.