Why only particular groups? That very question should defuse the racist generalisations which the media feeds.
My hypothesis would be that some immigrants are placed in areas with pre-existing cultures of disaffection and violence, and probably intergenerational poverty. It's no accident that the "African Gang" problem occurs where poverty exists, where people feel already under attack from a world that treats them as surplus labour without purpose, and see new arrivals as crowding their already full niche. It's a combination of alienated, atomised communities with no feeling for civic participation or social institutions and a lack of welcoming and integration that causes the young men of these groups to fall back on chauvinism and raw masculinity, as it is manifest around them.
Remember when all those Indian middle class students were bashed and robbed by white not-gangs? Same problem: outsiders placed in unwelcoming areas, and the students were perceived as a unjust gentrifying presence - leapfrogging locals on the symbolic ladder. Eventually the nation took notice when the students protested in Flinders St, and politicians realised that it would affect the current account balance (because foreign students are an export industry). African immigrants don't have that leverage.
In my town there are concerted efforts by local people (not govt, btw) to help recent immigrants integrate and for refugees to acquire needful skills. As a result, the Sudanese here are happy, gregarious, positive members of local society. I would argue that this is not merely anecdotal or exceptional (as these anecdotes can be multiplied around the country), but rather the incidents of violence are exceptional, and are evidence of a failed community more broadly.
