I'm giving a generic answer here. perhaps someone can make it more C90-specific.
There are two rotor systems in the PT6 (& many other engines), and each rotor is subject to its own failure modes. For example, if the prop fails to flat pitch, the free turbine is unloaded and can overspeed at a cruise power setting. In this case, the prop governor is out of control, and a very rapid fuel cut is necessary to keep the free turbine or prop from destroying themselves.
And there may be modes in the main fuel controller wherein the core rotor overspeeds, so a redundant o/s protection may be needed.
I'd think a good PT6 pilot fam course will fill in the blanks.