As far as politics, I know my promotion to major was held up due to politics. The Army does not have quotas per se, but instead has goals. In other words, the Army wants to promote officers in the same percentages as the population in the Army.
For instance, if 75% of the eligible officers are white males, 15% blacks and 10% women, it has a goal to try to make sure that the promotion list includes at least 15% blacks and 10% women.
The year of my promotion (1999) the selection rate was one of the lowest (77%) of all time. The Army was trying to downsize and basically trying to cut a quarter of the officers in this rank for that year group. But as the line was being raised, a disproportionate amount of the minority officers were not making the cut. Think about the 1 to whatever number on the list, let's say 1000. So the cut off list was the #770 person with the #771-1000 officers being passed over for promotion.
So in order to try to meet the goals, the list is kicked back in order to be scrubbed to make sure the percentages are correct, that any of the names of #1-770 aren't dead, resigned or committed a crime or did something else to get taken off the list. There are tales that some minority officers were bumped above white males who were more qualified in order to meet goals.
The promotion list is normally released at the beginning of the fiscal year (October 1st) but was sent back at least twice and the list was not released until late December and promotions began on January 1st.