People will hack the sites that are proxied behind Tor .onion domains. Hacking a site is also a crime, so you won't find too many documents revealing how a crime network was exposed. Instead, they gather data from the hacked servers and hand it over as anonymous tips.

Tor is just a proxy. A fancy proxy that uses onion routing, but a proxy nonetheless. Think of it as a CDN that does not have to hand over customer details because they have plausible deniability about who you are. Unlike commercial CDN's however, Tor does not try to prevent hacking. That exercise is left to the people running the servers that Tor is routing to. As it turns out, some criminals may be lazy or inept.