Human Factors. Period. Primarily crew mishandling the aircraft and in some rare cases maintenance errors. You could go back 40 years and the answer is the same.
Major malfunctions leading to hull loss are very rare by comparison.
This does not mean that we should be scared of humans in aviation just that the incredible advances in aircraft technology have exposed the human in the loop as both the strongest and weakest link.
If you want to review a textbook Human Factors accident, Air France 447 is the one. A perfectly serviceable modern fly by wire aircraft is stalled into the sea from cruising altitude. Tragic. A chain of errors that was too complex to unravel in the available time.
Conversely if you want to look at an example where creative thinking and professionalism came to the fore in the most extraordinary way (no I don’t mean Sully) where everyone should have died, look at United 232. it is extraordinary.
Thealready had been aggressively promoting their “Approach and Landing Accident Reduction” (ALAR) initiative for some time when CAST started its work. However, CAST wanted to make sure properly targeted efforts were made toward all of the issues underlying fatal accidents. So, they subjected each of the “tall bars” in the Pareto chart to separate “joint safety analysis team” (JSAT) studies, which were then translated into proposed courses of action by follow-on “joint safety implementation teams.”