What did trigger the explosion on Illus/New Terra in Cibola Burn?

Question

What exactly triggered the fusion reactors explosion on the other side of the planet?

Answer 1

Everybody assumes it was a failure, or otherwise an accident:

"Like a bomb?" Carol Chiwewe said.
"Or an alien power plant failing out," Murtry said. "Can't really speculate."

And:

The first-report newsfeeds were already speculating that it had been a fusion core overloading.

This theory seems to be corroborated by the response of the rest of Ilus's native technology:

"Our creepy friend said there was a defense grid," Holden said. "Their power station blew up, so the old defenses are in lockdown."
"They do seem to dislike big energy sources near their stuff," Alex replied.
[...]
"So we just need to get everyone to shut their reactors down like last time?"
"Seems this defense network has a new trick. They took care of that problem for us. Made fusion stop working.

The "last time" Alex is talking about is of course the similar defensive response of Ring Station and the Slow Zone in Abaddon's Gate. This time around, instead of tampering with the maximum speed of large objects, Ilus's defenses somehow disabled all fusion reactors in the planet's vicinity.

"The shuttle with the relief supplies was shot down by the planetary defenses," Holden said. "Which seem to have activated when the planet blew up. And they're running their usual 'high energy is a threat' program."

Disabling all fusion does mostly make sense if it's a defensive response to one of their fusion reactors blowing up by accident. Ilus wants to stop it from happening again.

So why did it explode by accident all of a sudden? Well, Ilus is old, and its technology seems to be falling apart. Says Miller:

"Kid, I keep telling you. I'm a wrench. The defense network problem in the gate hub just happened to be a hex nut. I've got no control here. Not much anyway. And this system is falling apart. Half the planet blowing up might not be the end of that."

Ilus is a planet that has been heavily modified and turned into an ore processing station and refinery. It has been defunct for millenia, however, and seems to be slowly breaking down.