Mass immigration disrupts societies but that isn't necessarily a bad thing per se. Look to Canada for an example of where it works pretty well.
The problem though is mass immigration of specific categories of individuals who bring with them ideas or norms that are not welcome.
Now, the thing is that if the stats are to be believed, Germany and other countries have absorbed huge numbers of immigrants from areas/ethnicities that, again if the reports are to be believed, are similar, if not the same, as the alleged attackers.
So, going by the numbers, it is impossible to believe that all of them should be suspect, however that does not rule out the idea that there is a distinct culture of sexual harassment, at best, and sexual abuse that some of these folk bring with them. Again, my friend who worked in Cairo said this and this has since been corroborated by my female Muslim friends.
These girls are simply not surprised at all about the consequences of inviting lots of young men into Europe en masse from these regions/cultures who have peculiar ideas about the sexualisation of women and white Western women in particular.