This started as a Kotaku rumor, but one crucial piece was confirmed after it was published. Irrational brought in a Gears of War producer to help finish the game.

tl;dr:
- a "significant chunk of the core Infinite team is no longer working on the game", including producers, level and system designers, the art director and other Irrational veterans
- team lacked focus, dedicating months to stuff that was then cut
- a lot of importance was placed on multiplayer modes which were cut (coop and tower defense with miniature players fighting toys lol). The game may ship SP only.
- 2011 E3 demo "far from indicative of the actual state of the game", some of the multiplayer team was brought in to help finish it
- the demo was made while "the team was wrestling with the most basic elements of the game, from the story to the level design to the AI programming to how to design the sidekick character of Elizabeth, who can magically conjure weapons and in other ways affect the environment." (LMNO anyone?...)
- Gears producer Rod Fergusson was hired to help finish the game



This sounds a lot like what the first BioShock went through - well, except for so many people leaving, so much money spent and no games to sustain the team while nurturing the impossibru project.