This subject is something of a scared cow... Since fatalities occur in many of the wrecks, we all want to turn a blind eye from some obvious pilot screw ups. Or.. the surviving pilot is our buddy so we only talk about what a great pilot we think they might be...
While there are a few incidences of true mechanical failure, most of these pile-ups can be traced to pilot goofs. Sometimes not just one goof, but a series of mistakes that lead to the eventual accident.
I see a couple things which always play a key role.
1. WEATHER.... The weather does not go out looking for you... YOU decide to fly in the weather which is prevalent in the area being flown. Many of these wrecks occur because pilots insist in flying in bad winds , rain, fog, snow or icing conditions. Some 135 pilots do it so they won't loose their jobs. Other do it because they are low time pilots and fairly clueless. Many do it because their EGO won't let them stay on the ground.
2. Exceeding the skill and experience level of the pilot.
a. This occurs when a pilot has been flying a lightweight plane with just themselves and some gear for the last few years.
Suddenly they decide to haul all their buddies and a bunch of gear out hunting. The pilot lets buddies Billy-Bob and Clitus convince them that as long as the plane has room inside, it should be able to take the weight.
And once again the pilot's EGO raises its head and says " What the heck, lets go....we certainly cannot admit that this might be making us a little uncomfortable.."
b. This also occurs when a pilot thinks that making short landings and take-offs at sea-level ( Lake Hood Strip) should count as experience for landing at a 3,500 feet high dirt and rock, sloping runway up in the hills...
c. In some cases it is because the pilot has never expanded the training/experience they receive. Having 2,000 hours does not mean squat if it is the SAME 2,000 hours over and over. These folks have log books full of pencil whipped BFRs and hundreds of flights along the highway using a GPS.
I will get off my soap box and let somebody else add to the list...