Human Error
Despite what a lot of people think, half of the (quite a few) aviation accidents are caused by human error alone. This means that, generally, the pilots have made some mistakes, which in turn have caused the accident. Be it that they were not properly instructed, too cocky, lacked experience in tough situations, or they were exhausted, there are a ton of reasons why pilots have made such errors.
The bad thing is that you never know when you’re on board with such a pilot, despite their numbers being very low. After all, they go through rigorous training, and anyone can be allowed to fly a plane full of hundreds of passengers. Moreover, despite some accidents being caused by a pilot’s error, they are also the people that can land you safely despite the problems. And more often than not, they solve the problem with no casualties.