A few notes:
Ostracism is traditionally attributed to Cleisthenes, as mentioned in Aristotle's writing on the Athenian Constitution, which would put it around 509 B.C. or so; however no records of its use exist until 487. The attribution is still in dispute, though it would fit with the general assessment of the purpose of the Cleisthenic reforms in distributing political power to the masses and breaking up the old aristocratic society. Which brings me to the second point,
Ostracism was an extension and refinement of the practice of exile used throughout Greece, and possibly already alluded to in Homer's Iliad, where in book 24 he speaks of Zeus distributing fates from his two jars, and pursuing the accursed in exile throughout the lands. Exile was a standard practice involving the removal of pollution from the city, used against murderers but also against a wide variety of other crimes. Which again brings me to the final point,
Ostracism was never bound to a particular crime; anyone could be ostracized as long as the requisite votes were met. If it was used as a political tool, it was as a convenient side effect; it allowed the citizenry as a whole instead of the select juries at the time to remove temporarily people they deemed to be of negative influence. There is a story in Plutarch of a country bumpkin voting against a certain Aristides simply because he was sick of his nickname, 'the Just'. It is understandable that textual records have left us only the names of prominent political figures, but excavations at the Agora have revealed some 10,000 individual sherds, most of which bear names entirely unfamiliar to us, and without mention of charge or reason.