AFAIK no employment laws or freedom to move rights for EU citizens in UK has changed yet?
So this is the effect of what people expect there to happen wrt. EU and UK in the future. Judging from the numbers the effect is quite strong but obviously also something that may change quickly once the arrangement between EU and UK after brexit is fully settled.
Even if no law has changed the fear of a hard Brexit might lead people to not risk their family to move to the UK and then been thrown out.
And yes, there's a possibility that a usable agreement comes out at some point in time, but by then companies might have decided not to wait, but move to the continent. Thus giving this a longer-term impact.
So this is the effect of what people expect there to happen wrt. EU and UK in the future. Judging from the numbers the effect is quite strong but obviously also something that may change quickly once the arrangement between EU and UK after brexit is fully settled.