Stupidity or clumsiness. Apparently it can't be tracked to any specific origin, the name just went completely out of fashion and the association seems to have emerged around 2000.

Here's a German article about it: https://www.welt.de/kultur/article163980590/Der-Herbst-des-H...

Basically the same as the other mentioned: Moron, idiot or dimwit. But the actual names used vary from generations and regions. Also I would say these name insults are mostly used by children and teenagers. As a somewhat effective workaround to avoid getting punished for using the actual swearwords.

Friends called their unborn "Horst" so that people (including themselves) had a name with which they could to refer to the future child, but at the same time the choice made obvious that it would not be the real name given after birth and that they did not want to disclose it beforehand.

I suppose that says something about the name's image.