This all started because they shut down the marijuana-market in Christiania. They shut it down due to gang-violence, which is only present due to the police raiding the place in the 00's, arresting all the locals and then expecting the issue to go away. To nobody's surprise organised crime, which had supplied marijuana to the market, now moved in and manned the stalls as well.
This wasn't too bad, but tensions have steadily grown, though the area was never particularly violent. Maybe the constant police raids didn't help, who can tell?

They tried shutting down the market several times, both with and without the help of the citizens of Christiania, but it never really amounted to anything. To nobody's surprise, whenever they shut down the area, you'd see a drastic increase in gang violence across Copenhagen, as they would now have to fight for territory. The danish police likes to claim that they are the ones who have ended these gang wars, but time and again it has been the gangs themselves that ended these wars by way of internal dialogue, usually after a particularly violent episode.

This dialogue has been facilitated by the knowledge that the market in Christiania would re-open, but this no longer seems to be the case.
So now we have escalating tensions between gangs in Copenhagen and the police aren't really doing anything, nor are they capable of doing anything, because they can't just rest on their laurels and let the gangs figure it out on their own, since the go-to solution has been removed. The police is used to doing some token action and then claiming responsibility for solving issues, which means it is expected they solve this, but they can't, because they don't actually understand why these things flare up or stop.
I fucking hate the cops, stupid corrupt motherfuckers