From Longman Dictionary of Contemporary Englishtouch something ↔ off phrasal verbSTART something/MAKE something STARTto cause a difficult situation or violent events to begin It was these national rivalries that eventually touched off the First World War. → touch→ See Verb table
Examples from the Corpus
touch off• But this was just a shrewd touch that came off.• An incident with a couple of heavy-handed cops touched it off, and the riot was on. 