Hi quadrirotor,
main cause - human.
Sure there is the one or the other mechnical or electrical failure - sometimes due to human errors, sometimes due to other causes.
But normally the pilot should be trained to cope with failures - even engine failures, thats why we train autorotations.
But doing low flying, planing direct ways with SE over water and/or woods, stuff like that (o.k., bosses want the short way) and the wrong reactions in case of a failure - makes an accident.
Show offs, poor preflights, short of fuel, forgeting to due the checks, hurrying into situations, lots of human causes ...
be aware, that the pilot is the greatest risk, and don't push yout luck to often and you might become an old pilot....
Greetings Flying Bull