The reason for this is actually pretty simple; Microsoft wants to narrow the cost difference between the Xbox One and the PlayStation 4. By adding in a major third party title to the bundle it makes the cost difference much less.
As for what title Microsoft is planning to bundle with the Xbox One, all we know is that it is a major third party title that was a planned holiday release. Some of the chatter we are hearing is that it will be FIFA 2014 in Europe and another title in North America. We will have to see how accurate this turns out to be.
Microsoft is claiming that they have an exciting announcement planned for Gamescom and they also plan to reveal another new game which is believed to be an Xbox One exclusive.