c.1374, “subject to or produced by chance,” from O.Fr. casuel, from L.L. casualis “by chance,” from L. casus “chance, event” (see case (1)). Of persons, in the sense of “not to be depended on, unmethodical,” it is attested from 1883. Casualty in the modern military sense is 1844, from the word’s older sense of “accident” (1423).

I suspect the use of casual here means something that happens by chance or accident (“A casual meeting”) … presumably meaning that the type of game depends on what happens to be available at port at the time?