I find this interesting, particularly in terms of "western values."
I'm of a multi-ethnic background, as is DH, and we have relatives in other parts of the world. Cultural disruption caused by large flows of males from one region to another is not confined to Western Europe by any means. It's causing quite serious issues in Turkey and impacting significantly on Turkish female behaviour in some areas (and many young Turkish women are not very happy about it at all).
My reading of the issue is this: whenever you have movements of large groups of males from a "home" space, where those males are within a context of social and cultural policing, to an "other" space, where that policing is largely non-existent, you run the risk of allowing a "Lord of the Flies" scenario. You are essentially enabling a situation where there is virtually no oversight of that male behaviour, apart from what the state can observe and because of the nature of these groupings, the state can observe very little.
That's your problem.
The question is not why these groups are engaging in these types of assaults, but why you don't see a similar level of related phenomena occurring with "native to an area" male groups. And that answer to that is social and cultural policing: in short, living within a context where people know you and who you are, and where you have a lot to lose, particularly if the result of a behaviour is stark social ostracism.
It's the same dynamic you see in young lads going bonkers in Magaluf, the whole Italian heroin London phase in the 90s, and the development of one-nationality criminal gang networks across Europe -- only this issue is also complicated by the scale of the various male migrations taking place, the background of differing cultural assumptions and preconceptions, and the difference in language etc.
The thing is that sometimes the dynamic works in a positive way. Bronski Beat even wrote a song about it, called Smalltown Boy. But what we are discussing here is the dark side of the coin.