"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.