Last week, Lorenzo Booker made the 53-man roster on the strength of his special teams contributions, but sustained a slight concussion in the preseason finale and hasn't practiced all week.
So maybe that plays a part into why Booker was waived today and Armando Allen promoted in his place to the 53-man roster off the practice squad. Allen will be the third running back behind Matt Forte and Michael Bush.
To possibly fill that practice squad void, the Bears waived Patrick Trahan. What this move also opens up is a roster spot to re-add Nate Collins, who was suspended for the first game of 2012. So, kind of an interesting cycle of activity that might not have much impact on Sunday's game unless the Bears plan to unveil a three-headed-monster rushing attack, but so it goes.
EDIT: According to Zach Zaidman, Booker is not waived, but has instead been placed on injured reserve. That's more like it, considering the concussion and all.