It is quite rare for an accident to be explained by one single cause. Almost every mishap is the consequence of a chain of events and accident reports usually discriminate between the main cause and a number of contributing factors. The following graph shows the distribution of main causes identified in plane crashes.
The main root cause is human error. In order to try and eliminate this as a source of accidents, crews are requested to follow a strict training routine. Next come aircraft failures, but these are less likely when it comes to modern aircraft.
Only airport security measures and widespread vigilance can protect from aeronautical terrorism (bombing and hijacking). Aircraft are very vulnerable to such attacks, because they are defenseless, and because a plane crash is newsworthy. Cargo bay containers are bomb-proof, but not aircraft, as it would make them too heavy to fly.
Fortunately, thanks to the many measures which have been implemented in a bid to prevent such attacks, the number of terrorism acts has dropped with time. When it comes to terrorism fatalities, however, there can be no trends. New aircraft carrying a lot of people may become targets of choice for terrorists, which could lead to extremely deadly attacks in future.