My Guess? Stack Ranking in action...
http://en.wikipedia.org/wiki/Vitality_curve
Many organisations have tried to energise their workforces through use of stack ranking performance metrics - all have failed eventually. One of the prime boosters of this concept was Microsoft, who only finally gave it up late last year: http://online.wsj.com/news/articles/SB10001424052702303460004579193951987616572
Stack ranking forces the concept of large scale dismissals on an annual basis because it presumes that there will always be a percentage of 'under performers' in any population of employees. In theory culling the drones out leaves space into which higher performing individuals can be employed improving the organisation incrementally over time. On the face of it that argument might not seem far wrong, especially if one looks at government departments where if you sacked all the under performers you might get down to the one person actually doing anything, (likely the cleaner).
But it is too simplistic, and fails horribly in practice since it creates an environment where internal competition, back-stabbing and brown nosing become the only means of keeping a job - performance suffers and the organisation either wakes up to itself, (have to give Microsoft small kudos for that even if it was very late), or it implodes.
If SpaceX is using Stack Ranking then it is a business in which I for one would not want to work...