Nice story! This gives proof, that not only settlement attacks matter, also wandering enemy NPCs do matter, as long as the cell is loaded. And it obviously was, so this happened. I can live with that, because essentially my character does the same! The enemies do respond more actively if you destroy one of their turrets, but they still do not know exactly where my character is. I find it actually quite 'realistic' and good. It was not the typical settlement attack, it was something else.
That the NPCs at the settlement do not react more actively is also clear - you can kill enemies with silent shots and even nearby NPCs will not react, since the only thing the really realize is the sound your character makes, shooting or moving. They do not react to one of their buddies being dead suddenly. And so Dogmeat does not react when a turret suddenly goes boom.
But a settlement which destroys itself, while you are there, without enemies is clearly a bug. If a sniper somewhere would shoot, you would hear it.
The entire problem is very old: In FO3 if you happened to come near enough to a trade caravan, so that the cell which they were currently was loaded, they often died. You would hear the shots, but arrive much too late to help. People who tinker with the load cell settings in the game (there are a lot articles about this starting with Morrowind and Oblivion) always risk to destroy their game, because NPCs died from fights, they should not have - simply because the cell was loaded and the player was very far away. But the game looks much nicer with more cells loaded, you can see much more in the distance. But also things happen...