Weather
The weather could have played a part in the crash. The Hindenburg traveled through storm clouds on its way to new jersey. Lightening could have struck the ship, but this was a regular occurrence for airships and the Hindenburg had been struck before itself.
It was suggested that the weather conditions could have caused a phenomenon called St Elmo's fire. This would have meant that when the guy ropes were connected with the ground, the static electricity built up during the flight through the thunderstorm would have discharged and caused a spark. This would have ignited any escaping hydrogen and led to the explosion.
This is not a satisfactory explanation though, as the guy ropes were connected from the front of the ship and the fire started much to the rear.