One part of the answer is that some are more open to out of EU immigration than others. A while ago that was Germany who got 1 million plus of refugees and economic immigrants from Syria , Afghanistan and other non European countries.
Now it seems that Spain is that willing to take immigrants and of course also provide them citizenship after 3-5 years. So you may end up with a disproportionate amount of immigrants that come from Europe but are not really European in terms of looks and behaviour. I really fail to see how EU will continue unless far right wins all over Europe and a common restrictive immigration policy is agreed.