When DNA is being replicated, it can only be replicated from the 5' end to the 3' end. Each strand is anti-parallel, meaning while one goes 5' to 3' in one direction, the other goes 3' to 5' in that same direction. As the strand unwinds, DNA polymerases catalyze the attachment of bases on the non-lagging strand continuously, in a 5' to 3' direction. However, going the other direction, you would have to leave the whole strand unwound or unwind it a second time in order to perform that continuously. Instead, small okazaki fragments are formed in the 5' to 3' direction, opposite the direction of unwinding (but continuing in the direction that the unwinding is occurring).
My professor discovered DNA Polymerase II... I guess that's more of a fact that a source, but I did learn this from him.