The motivation behind the label unergative stems from the fact that in an ergative-absolutive language, the only case which uniquely identifies a volitional argument is the ergative case, which marks the agent of a transitive verb.
Some languages treat unergative verbs distinctly from other intransitives in morphosyntactical terms. For example, in some Romance languages these verbs use different auxiliaries when forming the compound tenses. See the article on unaccusative verbs for details.
Besides the above, unergative verbs differ from unaccusative verbs in the fact that, in some languages, they can be passivized. For example, in Dutch: