When Alexander the Great died suddenly in 323 BC, his closest advisors and generals divided up his extended empire. As Alexander was in the prime of his military life at the time of his death, it is no surprise that he had mints all over the known Greek world striking coins to pay for troop support. Once Alexander was gone and the Diadochi quickly scrambled to take control of their respective areas, those mints needed to react accordingly. Seleucus I Nicator,one of Alexander's generals was initially not involved in the struggles of the Diadochi. He supported Perdiccas, another of Alexander's generals. Only after the struggle of the Diadochi did Seleucus become independent from Perdiccas and began to fight for himself. 

The first coins of all the successor kings showed imagery that was already understood and accepted as that of Alexander. The Herakles obverse and Zeus reverse was maintained but the legends after Alexander's death show the new ruler. In this case, Seleucus I. A novice collector might assume that this coin is a that of Alexander the Great but once you look closely you will see that the name of Seleucus instead of his predecessor.