People, especially those in leadership positions, are described as unaccountable when they behave as though they don't need to explain themselves, answer to anyone else, or take responsibility for their actions.
We often complain about the need for "accountability" when our leaders spend our money, oversee our wars, or make confusing new laws without explaining to us what's going on. If there's nothing in place to keep them from being corrupt, we say that they are "unaccountable." Another interesting definition of unaccountable is "unexplainable." Crop circles, Big Foot sightings, and unidentified flying objects are mysterious and unaccountable.