It appears that the thugs who carried out the assaults are "autonomists". Autonomism is a current of radical left-wing thought in Europe, embraced by both anarchists and some Marxists. The theory involved is described in wikipedia as follows:
Some of the most discussed issues in autonomous groups are the questions of self-determination, self-organisation and militancy ... In the understanding of the autonomists it is not ultimately possible to be autonomous (independent in the sense of being self-determined). Every person lives in a web of dependence, which is normal for a social creature. The main emphasis is on the question of how far these dependencies are other-determined or self-determined, the struggle being to live wherever possible without being other-determined ... The "triple oppression" (racism, sexism and class) belongs to the theoretical foundations of the autonomists ...
Reading this, I can't help but think that the autonomists are deeply confused. They have set themselves up as radicals and revolutionaries, but their ideals are the same as those of the establishment. They are followers of liberal autonomy theory, just like John Howard, Kevin Rudd and any other mainstream politician you care to mention. When the autonomists declare themselves to be in favour of voluntary social associations, rather than inherited or unchosen ones based on race, class or sex, they sound little different from your average Australian Liberal Party MP.
It is confused, too, for autonomists to support the growing influence of Islam in Europe. Do they really expect self-determination and autonomy to flourish in an Islamic Europe? The word Islam means submission, and it forms, as Omar Bakri Muhammed puts it:
a complete way of life that could not yield to any other way. “Islam is a complete system of living, the Sharia system. Islam has political beliefs — it cannot co-exist with another political belief.”
To underline this point, there has been a split in the German autonomist movement: one faction since 2001 has become critical of Islamic fundamentalism and suicide attacks - which is the more logically consistent position for an "autonomist" group to take.
So the Danes who were violently attacked ran into the wrong group of autonomists - those who, unlike some of the Germans, haven't figured out that Islam is incompatible with their own political beliefs.
In the meantime there have been more riots in European cities: in Amsterdam (see here and here) and in Brussels.