Refers to the Oklahoma Land Run of 1889 when settlers, seeking free land, made their way to Oklahoma to stake their claim to tracts of open territory. One of the few rules to claiming a lot of land was that all participants were to start at the same time, on the boom of a cannon. All settlers who started then were labeled as "Boomers" and the ones who sneaked in ahead of time were called "Sooners."
The University of Oklahoma named its athletic teams the Sooners in 1908 to pay homage to the cheating claim jumpers of the land rush. The nickname proved fitting as, over the years, Sooner athleteic teams have been the object of many NCAA penalties stemming from repeated violations of a variety of rules, particularly under the ignominious reign of football coach Barry Switzer when the already outlaw program sunk to outrageous lows.
What do you expect, they're sooners.
Some sooner stole my cash.