because all the spooks will be gathered in one place and for secrecy and expediency it's easier to swoop them all up at once in one shot - otherwise you need dozens of teams in place all over the country all trying to co-ordinate simultaneous arrests which can easily be mis timed and more people = more leaks

you grab some- the others can run or react or set off retaliatory actions etc.

so they swoop them all on the 20th.

hence all the troops - and hence all the efforts to maintain optics that they have lost and are standing down.