The Networks (which have rights to the league games) determine the drafting order of games, before the season. There are a handful of games, which are slotted before the season, these are usually the OOC games (we all know this, ELA posts these as they are known), then a few other conference games' kickoff times are set (but not necessarily which network), this is more recent trend in the B1G, as the league began to embrace more night games. Once that is done, these networks often utilize a 'six day' window (they only have so many to use), which really allows the top selecting network up until six days before kickoff to choose or select their game. The exception is the aforementioned night games. B1G doesn't permit six day window holds for night games selections
btw: Matt Sarzyniak has a great blog, and tracks all of this all year. Weeks ago he guessed Fox had the top choice, based on a comment made by Joel Klatt (analyst saying he'd be at the game). He wasn't clear on the time slot though. The Fox options were 11 or 3. As you all know Fox doesn't care about GameDay.