It's been mentioned in this thread (the evidence on which I base my assumptions). eg.Is there any proof that they were deep into development of a game that didn't even have a name? Or a game that actually had AAA franchise potential?
http://www.gamesradar.com/black-tusk-studios-looking-build-next-halo/
But I'll follow the paper trail to the very source, which was a story in The Vancouver Sun...
Built up from 5 EA vets to a 55 person developer before the renaming of the studio, so they were best part of a year doing that I imagine, and another year working with the E3 reveal until this change to Gears.
Select quotes, though you should read the whole article...
Seems excited at the prospect of doing their own thing.Studio staff have an average of 12 years experience in triple-A games, the industry’s equivalent of Hollywood summer blockbuster movies — “big games, big teams, big budgets, long timelines,” said Crump.
“What that means is we are working on Microsoft’s next big entertainment franchise,” he said. “We’re not working on an existing franchise, we’re looking to build the next Halo here in Vancouver, for example, which is really exciting. We are building something from the ground up.”
So they went through the work of refining their project, their exciting new IP, to get it green lit, get the go ahead, and had spent a year on it most likely, before getting taken off to do Gears. If in that situation Mike Crump said, "Yeessssss! Let's ditch our game and do Gears. Awesome!" then I accept I'm a lousy judge of human behaviour.“We have been officially green-lit by Microsoft executives to go ahead with the project that we’re on,” he said.
“That’s the way game development works — it’s just like making a movie, you go through pitching and concepting and then at some point you have a very large meeting with the executive team and present your full pitch with the business plan and you get the green light, so that’s happened for us.”
You then have some people questioning MS's position on it when Phil Spencer says the company's E3 showing was a concept piece and they've been incubating ideas for the past 6 or 9 months. That doesn't fit with the puzzle pieces, although there may be some pieces missing (like MS dropped the BT IP shortly after E3 and asked them to find something new to do, perhaps Gears, we'll look into it...). However, it's precisely the evidence that's fuelling at least my personal opinion. This is random, unsubstantiated theory.