does your forum have download or something in the name that could lead them/a scanning tool to think it's piracy download links? Not saying that would make the situation any more justified, I'm just kind of curious
The DMCA notices from Google direct me to the complaint in the Lumen Database. In those notices it lists my domain along with hundreds of others for each complaint, so I'm not alone here.
I'm assuming a third party company is being paid to look after sending DMCA complaints for businesses. If they remove 100,000 URLs by sending DMCA notices, they can charge higher fees or get more contracts compared to other companies that only take down 10,000 URLs. There are no repercussions, so might as well automate the process and aim for big numbers.