The official reason behind the delay is the usual rationale of polishing and tweaking "to make this the very highest quality game" the studio can deliver, and the Crystal D community is invited to ask Gallagher questions about the delay in this forum thread, which will be addressed in a podcast later on.
Getting Tomb Raider the hell out of the busiest season of the year is probably a more likely reason. While August is still relatively calm before any the E3 release date announcements, Square Enix -- Tomb Raider's publisher -- already has Sleeping Dogs positioned in the middle of the month. September features Far Cry 3 and Borderlands 2, and probably a million other games trying to escape the later months' madness.
Releasing it in October would pit Tomb Raider against Resident Evil 6, Dishonored, Medal of Honor: Warfighter, and Assassin's Creed III. November has Halo 4, Call of Duty: Black Ops II, and Square Enix's Hitman: Absolution.
Given that the last two years, the first quarter of the calendar year has seen a healthy flow of big games with plenty of breathing space between them, it's safe to say we'll have to wait for a February or March release.
Crystal Dynamics also revealed a shiny new screenshot of Miss Croft trying to hit a corpse pinata, "hinting at content you'll be seeing at E3," to keep everyone occupied for the next 3 weeks.
Tomb Raider release date shifts to 2013 [Tomb Raider official forum]
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