1) Pilots Error -- which is the most common cause worldwide. Misjudgement or faulty cockpit management on the part of the pilot(s).
2) Mechanical/structural Failure -- Engine quit or metal/structural fatigue. Improper/inefficient aircraft maintenance engineering will cause this.
3) Force Majure -- bad or turbulent weather conditions. Extreme icing can freeze aircraft control surfaces.
4) Erroneous/negligent air traffic controller advisories and miscommunication and lastly,
5) Hijacking/Terrorist attack -- no need to explain. The New York 9/11 tragedy still lingers in our vivid memories.