The government is cagey about how, exactly, this criminal activity was unearthed, noting only that Herrera "tried to access a link containing apparent CSAM." Presumably, this "apparent" CSAM was a government honeypot file or web-based redirect that logged the IP address and any other relevant information of anyone who clicked on it.
It looks like a combination of bad opsec and clicking on a download link.
I know there has been some back and forth whether it's good to use a VPN with tor and feel like this is just going to open up that conversation again.