Catastrophic Countdown

Why is Stuff Blowing Up if we still have two minutes left?

Thank you for activating the Self-Destruct Mechanism. This ship will detonate in three minutes. In the meantime, please enjoy some random explosions and blasts of steam. Human Resources thought it would make things more dramatic than a red light and a klaxon.

An area is exploding and falling apart, because a timed explosive has been set. Even though that explosive hasn't gone off yet, it's causing the rest of the area to blow up as well.

The bomb is usually on a short fuse. If the bomb dooms the villain, a Catastrophic Countdown ties in naturally with a Collapsing Lair. While usually this trope is serves the need to make danger visible, it is not unreasonable for a Self-Destruct Mechanism to destabilize before it obliterates.

See also: No OSHA Compliance, Made of Explodium. Compare Going Critical.

You have two minutes to read examples of this trope!

Film

Captain America: The First Avenger has the Italian HYDRA Base undergoing several minor explosions before resuming the rest of the self-destruction protocol-related explosions. Justified, as Zola briefly interrupted Red Skull while he was arming the self-destruct protocol when shocked that he's blowing up the base before Red Skull calmly informs Zola that "Our forces are outmatched" while pointing to a video screen of Captain America fighting the forces and then resumes inputting the self-destruct protocols.
In Aliens, the mining facility where the action is set begins to fall apart several minutes before the 19-minute countdown given by Bishop until the reactor's failure. In this case, there is overlap with Excessive Steam Syndrome as well.

Video Games

Metroid:
- Super Metroid plays it straight, twice. When Ridley escapes the space colony in the beginning, an announcement claims that the colony's self-destruct has been activated, urging immediate evacuation. Naturally, the whole place starts shaking, steaming, blowing up, and even rocking back and forth quite impressively. Later at the end of the game, the entire planet starts exploding and flooding with acid, because a "Time Bomb has been activated".
- See also, Other M. An AI voice announces over an intercom that a self-destruct sequence will detonate in about five or so minutes. For some reason, your escape becomes riddled with burning debris and wreckage.
- The first game is a bit of an interesting case. While the NES original completely averts it at the end of the game, the GBA Remake barely offers an appropriate Retcon for playing it straight in that same area. It does avert the trope later on though, in the pirate mother ship.
- In the final escape sequence of Fusion, this is averted. There is some destruction near the exit of the ship, but that's due to the Post Final Boss rampaging about.
Star Fox 64: During the Fortuna mission, Slippy notices several enemy craft leaving the captured base. ROB 64 informs the team that the enemy planted a bomb in the base, and before Fox can defuse it, he must deal with an attack by Star Wolf. If you take long enough in defeating them, you'll notice the base starting to spark and billow forth a few small explosions.
Halo 3 has a variant of this. After starting up the Halo Ring the ring needs several minutes to charge and literally shakes itself apart.The last section of the game is a mad dash to find a way off.While there's no explicit timer the structural plates fall off at a constant rate.Justified in that the thing was half-built when it was fired.(It was a replacement for one blown up in the first game.)
In the Sega Genesis video game X-Men: Clone Wars, destroying the Sentinel activates the self-destruct function of the entire Sentinel factory... and hundreds of smaller, random explosions prior to the one that actually destroys the factory.
Descent and its sequels. When you shoot the reactor in any of the mines, a voice announces a Self Destruct Sequence, and you have about one minute to get to the exit before the entire complex goes nuclear. For no apparent reason, during this minute the screen shakes uncontrollably while lights flash and sirens roar.
A countdown isn't actually seen in Rebel Strike, but the trope is present in spirit in the climax where Han Solo has to escape from the self-destructing Endor shield generator Bunker while fighting against Sarkli before the base completely self-destructs.
In Medal of Honor: Allied Assault, after planting the bombs, Fort Schmerzen is rocked by explosions that you have to stay ahead of as you race for the exit, although it doesn't completely explode until you get out.
The laboratory self-destruct mechanisms in the Resident Evil series do this.