Alarm clock

[lovejoy0120]

Why do people say 'your alarm clock has gone off 'when it has just switched on?

[Quizmonster]

Like so many phrases in English, 'go off' is simply an idiomatic usage. You might as well ask why we say 'mark off', 'pass off', 'show off', 'take off' and many other such examples. We just do.
There's the story of someone clinging to a bus door-pillar, leaping in and out of the vehicle as it travelled along the road. When someone asked him why he'd been doing that, he (a foreigner) replied: "The conductor was shouting, 'Come on...get off!'"

[mycats]

It's like, why do we get "off" a bus but "out" of a car??

[smudge]

Your logical question made me chuckle lovejoy!