One of the biggest reasons that small planes crash is due to the fact that amateur pilots will have fewer restrictions than commercial airline pilots. They do not need to have as many flight hours of experience behind them to get an amateur flight license, and inexperience plays a big role in the number of small plane crashes.
Another cause of small plane crashes is a loss of control that can be caused by turbulence and other natural hazards that are more likely to affect smaller planes. Some small planes have crashed after a bird flew into their engines, and the weather can also contribute to small plane crashes.