What?
Building directly on top of Employee cohorts and staff fluctuation, this feature would add short-term fluctuations to the mix. Things like sick-leave, time-off etc.
Consequently, a company would have to have more staff on payroll to have sufficient buffer to compensate for said fluctuations.
While seemingly simple and logical, the challenge lies in making this meaningful. Because this only adds any depth if not having enough buffer has any sort of consequences. At the very least, this would be negative impact on staff mood due to increased workload/over-hours (or the opposite in case of generous buffers). In a more elaborate model it would also have operational consequences: turnarounds taking longer, check-in queues growing larger, flights cancelled due to missing crew, and so forth. Ironically, automatic re-hiring of new staff might slow down as well 
Why?
This nicely capitalises on Auto-Ops and adds yet another “soft mechanism” to the game where players have to decide how much added cost they are willing to bear for an increase in operational reliability, staff satisfaction and consequently the airline’s image.
When?
As stated, this depends both on Employee cohorts and staff fluctuation and Auto-Ops / Aircraft Pools.