From the article (which is almost entirely an interview with him) the reason for his arrest was harassment, and one poster urged people to post stuff to the councillors and it seems some people did.
The timelines aren't clear from the article, but it seems that just posters weren't the problem. But inciting people to use the postal system to harass was. Intending your action as a 'joke' or blaming the people on the receiving end for overreacting are typical actions of bullies.
(PS - for any Python experts reading this, were any camera crew part of their creative team?)