What caused the Belgian aircraft mechanic who stole an F-16 to crash?
Unauthorized flights of military jets by mechanics have happened in the past.  Some of the aircraft landed successfully after the ordeal others resulted in fatal accidents.  The two successful cases which I’m aware of, the mechanics had previous flight training.  We still cannot determine whether this F-16 went down as a result of inability of the pilot to control the aircraft or whether it was an intentional suicide by airplane.

There is nothing outrageous or unusual about an F-16, save that it is a high performance fighter, with a high thrust to weight ratio, (relatively) high wing loading and low aspect ratio wings and handles accordingly.  The sidestick controller combined with the fly by wire flight controls and nimble handling requires a light touch on the controls - some pilots describe it as almost a ‘kissing’ sensation - to properly fly, which possibly could get away from a neophyte.  Like most high performance airplanes, it needs to be flown by the numbers and can be demanding to land, though not as difficult as some other military jets eg T-38, etc.

The physical act of flying seminal maneuvers is very easy in these jets with a little flying knowledge.  Most fighter pilots describe their jets as easier to fly than say a Cessna.  Flight control laws eliminate a lot of the babysitting and grunt work one has to do in an aircraft with conventional controls.  The pilot has an unobstructed view from the canopy of the environments, offering very good situational awareness.  All primary flight data is displayed prominently in the HUD and easy to decipher.  More complex flying will start to become demanding and potentially lead to trouble.

Aviation has a lot of scenarios which seem benign but turn into death traps if you don’t fully understand what’s happening.  There are plenty of sad tales such as stall spin accidents, low level aerobatics, etc. where people thought they could handle a situation but their reach quickly exceeded their grasp.  This is compounded by untrained human psychological responses to stress and fear.  Some of the lethal gotchas I’ve seen are inability to understand aerodynamic concepts like stall speed to load factor, density altitude, aircraft system interactions, emergency procedures, operation in adverse weather or IMC, jet engine thrust response time to throttle commands, etc.  You also have the unusual quirks specific to each aircraft.  The F-16, for instance, can enter a deep stalled condition lethal at low altitudes if not properly flown.  These kinds of phenomena exist on other fighters with neutral or negative dynamic stability as well.

The exact cause of the accident in question is not totally clear, so anything specifics would be speculative.  If someone can find the Norwegian AF accident investigation report on this, we can read it and see what their conclusions were.