Meaning of “set sth off” - Learner’s Dictionary

set sth off

phrasal verb with set us uk /set/ verb present participle setting, past tense and past participle set

to make something begin or happen, especially a loud noise or a lot of activity:

He's always burning the toast and setting off the smoke alarm.

(Definition of “set sth off” from the Cambridge Learner’s Dictionary © Cambridge University Press)