The Nr Fairy
Off the cuff I can think of three possibilities.
1) The governor failed. Wouldn't be the first time. It's a solid-state computer so it could easily be some spurious electrical fault that can't be duplicated during troubleshooting. See the safety bulletin about not flying close to transmission masts. I've had a governor that kept "hunting" for the right RPM, constantly surging by 1-2%. A new controller solved that.
2) Awfully unlucky timing if you bumped the governor switch off. If you keep your left hand on the throttle grip (as you should), you'll notice that with the technique that you describe (governor ON at 75%, roll up to 80%, governor takes over and puts RPM at the top of the green), the governor rolls the throttle off by a small amount as the RPM goes through about 97%. If you swith the governor off before this point an overspeed might result.
3) The governor got switched off with RPM at 104%, but then the collective creeped up (quite possible depending on rigging if the pilot doesn't hold it down). The mechanical correlator will then increase RPM quite a bit (nearly had an overspeed like that once).
If you let the governor bring the RPM up as you describe, it's a good idea to hold the throttle grip so as to prevent the governor from winding it open as far as it wants to. I hold it to keep the manifold pressure below 15" (near sea level). At low RPM the tail rotor is nearly useless, so on a slick or icy ramp the torque from an enthusiastic application of throttle by the governor (or pilot) can spin you around despite full left pedal.
t'aint natural
As I understand maintenance requirements following an overspeed: between 110% and 116% (over redline but still on the scale of the tach), simple inspection of both engine and rotor system, possibly also check tail rotor driveline runout. Over 116% (off the scale): expensive engine teardown inspection, several engine parts scrapped regardless, magnafluxing of several parts of the rotor system, main rotor spindle bearings inspected, and definitely check tail rotor driveline runout. Expensive and time-consuming.