I am well aware of why but when I checked the seniority list I noticed I have moved some 14 places within the last 5 weeks. I am within the top 500 and it doesn't normally move that much, taking account of the fact that this is most likely a group that are here for the long term.
One can only imagine what is happening further down the list. This recent movement will not have been influenced yet by the
pay rise 
or the new rostering system
Interesting times ahead