Failure to integrate is the right answer. At some point immigrant children ceased to have proper access to proper socio-economic status and a parallel society developed which, Scandinavians being Scandinavians, the majority ignored. It's been a difficult time for the prospects of youth in general but that hit the immigrant population way harder as they're not as embedded in the local social network, no "cousin of a parent owns a repair shop he'll give you a job and tide you over".

Active xenophobia isn't even needed, all that's need is a failure to see and care. It's also generally a urban problem, both because not enough care was taken to encourage immigrants to not be urbanites (a common bias with arrivals is that "city is where the jobs are, rural areas are shitholes" which isn't at all true for Europe in general), as well as urban society generally being ass at reaching out to people, smaller places are way more tight-knit.

Of course, with shit having hit the fan xenophobia then becomes an issue of its own reinforcing the very issues that caused everything, and down the shitchute we go.