Early in the film Star Wars: A New Hope (1977), in the blast chamber of the Death Star, we learn how, after Vader orders "Commence primary Ignition" to destroy Leia's homeworld Alderaan, the Imperial gunners begin the ignition sequence by pressing buttons with switches on a panel of lights and pulling control levers in front of a bank of lights on a wall.
Then, near the end of the film, we see again the exact same ignition sequence, but this time, just before the planet-shattering superlaser is fired to destroy the rebel base in the fourth moon of Yavin, an intercom voice can be clearly heard saying "stand by".
The Star Wars A New Hope script contains two intercom lines in the same scene: "Stand by to fire at Rebel base" and "Standing by".
Then the Death Star explodes.
I have found some explanations as to why firing the superlaser was put on standby.
The first one relies on the short time elapsed between its firing at Alderaan and the Battle of Yavin as the cause of the "standing by". The massive superlaser might not have been at its full power as described in Star Wars the essential guide to weapons and technology.
The Death Star's power core needed an entire day to build up enough charge for a full-power blast.
A second explanation could be that the Imperials who analyzed the Rebel attack and learned that the station could be in danger and sent the information with officer Moradmin Bast to Grand Moff Wilhuff Tarkin, needed more time to take corrective/preventive actions.
A third explanation is found in the legends where the Death Star gunner Tenn Graneet, who was the responsible to push the last fire button, froze, broke down, wracked with guilt when received the command to fire, said "stand by" as described in Wookieepedia legends.
When given the order to fire the superlaser at the rebel base on Yavin 4, Graneet froze, wracked with guilt and wishing desperately for something to save him from his dreadful duty. In desperation, he informed his gunnery crew and the overbridge to 'stand by' and then repeated his order a few seconds later, until his wish was granted, as his few moments of hesitation were sufficient to allow Luke Skywalker to destroy the Death Star. Graneet died in the resulting explosion. Incidentally, his hesitation also allowed the Rebels to eliminate not only a significant force of manpower and materials fielded by the Empire, but also some of the most achieved and capable of the Empire's officer corps, including Grand Moff Tarkin.
A fourth explanation might well be found out-of-universe if the 'standing by' was a cliffhanger plot device used by the film's producers.
A fifth explanation is: Sabotage, probably by Obi-Wan Kenobi before his duel with Vader, that made the superlaser inoperative. How come?
In the film, during the Alderaan ignition sequence, the time elapsed, between the sitting gunner pulling down the lever and the hissing/humming (and shot) of one of the eight focused kyber crystal beams, is less than one second.
During the Yavin 4 ignition sequence, after the same sitting gunner pulled the same lever, there is no beam hissing/humming at all during the next 3 seconds, but only a pensive Grand Moff Tarking and the black space surronding a solitary Death Star before its explosion.
In other words, the superlaser never fired on time.
I am interested in learning if there is a canonical explanation for the why firing the superlaser was put on standby shortly before the demise of the DS-1.
Update: To support a bit my fourth explanation, according to Lucasfilm creative executive Pablo Hidalgo in his 2004 audio commentary of A New Hope:
"The whole plot point of the Death Star being in the Yavin system, and being 30 minutes away from destroying the Rebel base, was only added during post-production. That ticking clock element was not in the script and was later needed to create tension for the final act".