The most relevant part is at the very bottom of the article:
There have been reports in recent months of small explosions across the state of North Rhine-Westphalia (NRW), where Cologne is located, due to gang-related activity.
In the Netherlands, setting off small explosions outside buildings is a common intimidation tactic for some drug traffickers. NRW borders the Netherlands and is on the obvious route from there to central and eastern Europe, as well as the rest of Germany.
That coupled with the drastic reduction in opiate scripts helps. The hardcore have died or are finding treatment, the interested are finding little supply to start the habit, it’s all around a good thing to have fewer opiate painkillers in use. They absolutely have a place in society but that place should be “everywhere”.