its from the old wagons used to cross the prairies by the pioneers...here you go...

In 1889, the Indian Territory was opened to settlers. Thousands of people lined up on the border and, when the signal was given, they raced into the territory to claim their land. Some people went in early to claim their land. They became known as Sooners.
Hence, Oklahoma's nickname today is “The Sooner State”.

Grant me the senility to forget the people I never liked,the good fortune to run into the ones I do,and the eyesight to tell the difference.