Unlike most larger racetracks, the pit area at Marysville is located outside of the racing surface, so if Johnson was unable to turn, it could explain why his car went straight into the pits. A witness' statement in the some news reports indicate that the car may have had a stuck throttle, but there has been no definitive information released as of yet to say exactly what happened. The track's website states that it requires everyone in the pit area to be at least 14 years old.
This isn't Marysville Raceway Park's first fatal incident. In August of 2010, two competitors crashed during a semi truck race, killing one of the drivers. Scroll down to watch a couple of local news reports about the tragedy.