Basically when you use tor, tor normally makes a circuit consisting of the client, 3 nodes, the server. However the client, server and each node, each have an isp, isps log everything they do, they know to who they send packets and from who they recieve packets, so basically the government can traceback to the client by getting logs from the isps of the each node and server?
Edit: I mean more like correlation attacks, For example if you hack a server using tor, the server has the ip address of your exit relay so law enforcements can get your exit node ip address from the server then they check your exit node isp to see to who this ip address was communicating at the time the server got hacked, then they get your middle node ip address from your exit node isp, then they do this to your middle node isp and vice versa untill they find the client.