The root cause appears to be that engine was not "cut" once it had increased to an un-required/in-demanded power setting in excess of what was commanded/required.
I have an ACS keyed ignition switch (Off L R Both Starter) on my VLA, but I sometimes wonder whether it is a safe arrangement. During power checks, unless I am feeling my way through the detents or looking at the key, it is difficult to know if the left, right or both mags are on. There is also the need to ensure the key is not turned too far, to avoid engaging the starter with the engine runnning.
Although this keyed switch is quite common on VLAs, I wonder if the simple red starter button and lever mag switches would not be a better arrangement, especially if the mags need to be cut in a hurry.