Controlled flight into terrain can cover incidents such as flying in limited visibility using instruments where the instruments aren’t wrong, the pilot is conscious and in control, but gets task focussed or misinterprets a signal and takes deliberate action that ends up in an unplanned lithobraking event.

Flying limited visibility or with horizon obscured is a typical scenario for controlled flight into ground. Some pilots will also feel less confident with a particular implementation of the artificial horizon.

Then there are transition states between particular instrument assisted flight modes (“autopilot”) and manual flight modes, as well as assisted flight modes where the pilot and the computers disagree on the appropriate course of action (similar to, say, automatic trim of horizontal stabiliser in aircraft with engines slung under the wing). The pilot might switch modes and assume control over pitch and yaw but the computer was handing over control of throttle. So the pilot ends up fighting the computer for control rather than actually flying the aircraft.

Labelling it “pilot error” can cause investigators to stop looking for the root cause. “Controlled flight into ground” leads to the question “why would an otherwise competent pilot do what this pilot did?”

"Controlled flight into terrain" is a euphemism (one of several) for "pilot error". It's a term of art in the aviation safety industry for "the aircraft didn't break, it wasn't in an unsafe state like a dive or spin, the pilot just flew it into the ground".

In this case, though, it seems to be used incorrectly because it's being applied to a problem that causes the aircraft to be difficult to control. A plane that is too hard to fly isn't the pilot's mistake.

> Presumably no pilot will fly into terrain if in control.

The history of flight has many instance where this has happened.

A very common cause is bad visibility mixed in with mountainous terrain.

Controlled flight into terrain used to be one of the leading causes of aviation accidents. All it took was for the pilot(s) to get disoriented, distracted, or make a bad miscalculation, particularly at night or in cloudy weather.

Ground warning sensors and training are better now so there's less of it, but you still get it from time to time.