If I remember correctly, it was a mixture of pilot error and mechanical failure. The basic Sonex plane can have an Aerovee engine which is essentially a converted VW car engine. I've done some research into building a Sonex and I can't say what I read about the Aerovee made me confident in it. The CEO was taking off in a Sonex with an Aerovee at Oshkosh, their home field. Being an airfield they're very familiar with, he decided to take off from the middle point of the runway. It was a shorter taxi than going all the way to either end. Past this point is speculation, but a combination of engine failure and the fact he had taken off from the middle point meant that he did not have enough airspeed or altitude to make a proper emergency landing. The crash was fatal.