The reason why this happens is because Muslims are over-represented in city-centers, especially the largest cities. So the people who live in those cities get the false idea that the overall Muslim population is higher than it is, because around them a 15-20% Muslim population is reality. If you ask the rural folk, they've never even seen a Muslim in their lives, but even then they might be fooled into believing that there are many of them due to reporting by the media, and their over-representation when it comes to newsworthy incidents.
The actual violence will hardly even occur until these countries reach a 15-20+ % minority population. At that stage, the full-scale conflict between majority/minority cultures becomes inevitable. Why we are seeing this in major cities, specifically (and never in the more rural areas), is because the cultural struggle is already present as a result of the 15-20+% Muslim populations there. In areas where Muslim population have gained the majority (such as Malmö in Sweden, for example), there is less and less room for non-Muslims to inhabit, while the violence and crime is frequent enough to have become a daily feature for any of the citizens living in the area.
One culture strives for dominance against the other, and is atleast temporarily more unified in their cause, in the face of a divided "majority" of natives, who struggle to come to agreement about anything. That's why, despite smaller (yet ever growing) numbers, they represent a threat to the majority population. One can easily see the trajectory for the next century or so, if the same sorts of policies towards immigration and multi-culturalism continue.
