This, and more.
While Tor is generally safe, the way it works is you connect to other users/servers called nodes, and you never know who exploited what, if you're running appropriates configurations and if you're safe or not. You might get your PC infected by the most advanced botnets that will use your PC for illegal activities (DDoSing, password cracking by brute-force, etc), you might get your personal info stolen, there's a whole load of stuff that can happen if you happen to go to the wrong place or if someone manages to get a hold of your info or computer. And most of this happens without your knowledge, and when you realize, it's generally too late.