Steam redownloading full game.....

try this http://gaming.stackexchange.com/a/165030

Has happened to be that a MByte sized download/update causes a copy of the game to be created which is GBytes in size... (Not necessarily GB downloaded, just HDD space eaten while it lasts)
Odd things happen, but I'd let it to its thing even if stupid

Try deleting the .exe file, cancel the download then restart Steam then try download again

odd and im not sure entirely what set it off to start the download over again, but i have noticed that if a game (any game) updates separately thru a launcher or various other means and then you either go to back it up with steam backup system or verify the files it will look unoriginal to steam and start to redownload chunks of the game until it matches back up with the steam version even though the steam version might of been older and more outdated then your client side already was..

so in those regards if GFWL was altered or pulled from the game (by you or by valve themselves) then that would likely cause the trigger to redownload.