TDR issues are mostly software - but sometimes hardware - commonly driver(s), system files, heat, perhaps a combination of issues creating a logjam. Sometimes, the reason should be obvious - you've just installed a new graphics card/driver or it only happens during a specific game. Other times, it's a case of checking everything, uninstalling all but the basics, vital components and software, checking, testing and rechecking all hardware.
It can be a(nother) real pain to figure it out.