The Athenians, being the untrusting souls they were, checked the accounts on a regular basis. On one occasion, the numbers didn't add up. The ten treasurers were instantly charged with embezzlement.
This is what happened, from a surviving court case that mentions this unfortunate incident as a precedent:
Then again, your Hellenotamiae were once accused of embezzlement... Anger swept reason aside, and they were all put to death save one. Later the true facts became known.
This one, whose name is said to have been Sosias, though under sentence of death, had not yet been executed. Meanwhile it was shown how the money had disappeared. The Athenian people rescued him from the very hands of the Eleven, while the rest had died entirely innocent.
The Eleven was the official Athenian body responsible for carry out state executions. In other words, Sosias had been in the hands of his executioners when they retrieved him. The implication of the "it was shown how the money had disappeared" is that it was a mistake in the books.
So the other nine treasurers died for an accounting error.