Why do they say "an alarm going off," if it is really going on?

Answer 1

Because they say we drive up the street and down the driveway. Also because when a house goes up in flames, it burns down.

Just think about it. Would you rather be up in arms, or down to earth? Is a doornail really dead? Maybe it's because their brain is stuck in first gear, or the elevator doesn't go all the way to the top.

As a rule of thumb, we usually speak with our tongue in our cheek. When the chips are down, the situation is up for grabs, and some well-heeled opponent is tossing us a red herring, we must knuckle down, hold the line, call the shots, hit the bull's-eye. You want to be on a roll, get the ball rolling, take the bull by the horns with no holds barred, and put the ball in the other guy's court. Other wise, we may end up jumping the gun, not up to par, down and out, out in left field, behind the eightball, barking up the wrong tree, taking the bait, hook, line, and sinker. Then we will have to face the music.

Answer 2

The Alarm being on means that it is armed and set - ready to go off.

The alarm going off means that it is not in complete readiness because there is an error -> an intruder, a problem.