hm.. not sure but perhaps if a module takes more damage than needed to destroy it, the excess damage is transferred to adjacent modules as direct damage? i often see in battles that buildings can have chain collapses even if adjacent pieces had not taken much damage - although ive never checked to see if similar things happen in ships..

Ahhh, so its not fire. I always thought it ws fire.

Interesting mechanic. Thanks for your service to the community. This is a very interesting bug/feature, and one that may impact higher levels of play.

@ak31415 Thank you for the theory.
HP bonus is maximum with two solid sides, so Chamber has full/max HP bonus even without Spikes. And it was unscratched with armour intact, 100% HP.
Test show that even full (almost) HP Cow-catcher Ram is fully destroyed upon decomposition of Spikes attached to it:
https://psteamcommunity.yuanyoumao.com/app/342560/discussions/0/3393916911752990743/
So it seems that decomposition damage is another mechanic, not well known.

Just a theory.

Yes. This is because, as you know, modules being next to other modules increases their HP. Imagine a suspendium chamber has, 150 base health, + 10 health being added due to another module next to it. In this case, the spikes. So if the suspendium chamber is at 10 hp, and then the spikes are shot off, the +10 hp is no longer being granted, which causes the module to instantly die.
Here, multiple HP giving spikes chain reactioned, which caused a bunch of insta damage.