He and his lawyer started doing that in the late 90s. When the labels just started knowing what the heck this newfangled internet even is.

In den späten 90er Jahren, kurz nachdem er wegen Kreditkartenmissbrauch verhaftet worden war, wurden in Hackerkreisen Stimmen laut, dass Kimble auch seine Freunde an die Softwareindustrie gegen Entgelt ausgeliefert habe. Der damalige berüchtigte Schwarzkopierer-Jäger und Anwalt Günther Freiherr von Gravenreuth hat seinen eigenen Aussagen zufolge Kimble als sogenannten Testbesteller beschäftigt.

Source

Just one of the articles that goes into this assholes history

Translation

In the late 1990s, shortly after he was arrested for credit card fraud, there were rumors in hacker circles that Kimble had also handed over his friends to the software industry for a fee. Günther Freiherr von Gravenreuth, the notorious pirate hunter and lawyer at the time, said he employed Kimble as a so-called test orderer

They mass-collected the connection data and started mass-sueing/extorting. Nobody else did on such a large scale at the time.