That trick was in the movie "The Worlds fastest Indian" Great Movie by the way Based on a true story with Anthony Hopkins.

Preview of The Worlds fastest Indian

The life story of New Zealander Burt Munro, who spent years building a 1920 Indian motorcycle -- a bike which helped him set the land-speed world record at Utah's Bonneville Salt Flats in 1967.

Sodium reacts with water in an exothermic reaction. Thus the ignition. It's just what happens.

I don't think Sodium occurs on its own in nature at all, which is why you don't see fires on the salt flats every time it rains.