Synonyms for Causeless (same or very similar meaning)
WordNet sense 1 (having no justifying cause or reason):
reasonlessWordNet sense 2 (without motivation):
unmotivatedWordNet sense 3 (having no cause or apparent cause):
fortuitous, uncausedWordNet sense 4 (not deliberate):
unintendedFrom the ODE community, based on WordNetadd/edit