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In a sense, one might say that a supercontinent is actually the "natural state" of the system, and the present situation where the continents are spread relatively far apart an anomaly. All the southern continents (S. America, Africa, Australia, Antarctica and India) were essentially a single block from ca. 750 Mya when Gondwana formed all the way to the breakup of Pangaea around 150 Mya, about 600 million years.Originally Posted by PaGaN
Also, it should be noted that even Pangaea was never really unchanging, or even composed of a single continental plate. Eurasia kept amalgating pieces of Gondwana (Amurian plate, Cathaysian plate, Cimmerian plate, South China plate) all through the lifecycle of the supercontinent, starting off with basically just Europe and Siberia, and ending up with the present day Eurasia (eastern Eurasia is also very mountainous as a result of those numerous collisions). All that geologic history shows a cycle of pieces breaking off from Gondwana (due to slab pull), with associated opening and closing of the Tethys Ocean (Proto-Tethys, Palaeo-Tethys, Neotethys and finally the Indian Ocean, the latest stage of the process). This suggests that slab pull is probably the most important factor (though not necessarily the only one).
However, it is important to remember that ultimately the renewal of oceanic crust is what drives the whole system. Continental crust is relatively stable, but oceanic crust renews itself on a cycle, and that is what fundamentally creates the Wilson Cycle (it might be more accurate to consider it as the "ocean cycle" rather than the supercontinent cycle). Well, mantle dynamics probably play a role as well, but it's still fairly unclear exactly how the relationship between plate tectonics and mantle dynamics works (and in any case, I don't think you need to pay too much attention to mantle dynamics when creating a fictional world).
