The degree of ethnic homogeneity did not increase during that time, if anything it slightly decreased (or significantly in some major cities).
One possible explanation is that people willing to engage in low level crime simply moved to richer western countries because well they were (and still are) richer... e.g. in Norway Lithuanians are the second largest group of people who are imprisoned (after local Norwegians). The situation is similar in some other Western European countries. I'm not an expert but if I wanted to rob/steal from people and businesses I'd probably do that in Norway, Germany, or Switzerland rather than Romania or Lithuania. The risk versus reward ratio seems much better there. Also there prisons are way nicer (especially in Norway).