Is it my imagination / did I miss it or does that article really not venture to answer the question in any kind of scientific way?

No. They are all in the most deprived areas.

Nope: Decade of public spending cuts, crazy regional and national inequality, and right wing agitprop from the major political parties and media etc.

Yeah I'd say the misinformation about the original crime's perpetrator was the main cause for all these issues. There were already a lot of worries about immigrants and asylum seekers, which had been weaponised by a far right wing media and online fake news sources, and the association of the recent attack with someone from that group just lit the spark that set it all off.

Could drugs have had an effect here? Maybe, but I suspect the same issues would have occurred regardless due to deep seated issues with extremism and misinformation in society.