Fallout – What affects a trader/store generating caps

What seems to be the problem is that you haven't assigned the beds to anyone, which makes someone wanting a bed to rest look for the closest one and if it is occupied already, makes that person unhappy.

This is based on those 2 threads I found talking about the same problem and that the solution was to assign the beds.

First one :

So I took the time to put it to where I had exactly 20 beds, and then assigned each person a bed that night. It's been a couple days in game, and they haven't complained since.

Second one :

I've had some issues with beds as well. My latest problem was fixed by manually assigning everyone beds in the workshop mode (hit E on settler, then E on bed). Maybe try that.

I'd placed beds in every house in Sanctuary at the start of the game, thinking people might use them eventually so why not. But the settlers picked random beds all over the town, and something about that situation was making them unhappy. So I consolidated the beds into the houses in the center of town, made a town bell to gather my settlers, assigned them all to nearby beds, and put the unused ones into storage.

Your settlement was infiltrated by Synths. If you look where the bang happened you'll probably find a dead settler who has synth components on them, or if a settler just started randomly shooting then they're the Synth. I had this happen to me as well, when the Castle was attacked by Super Mutants. Turned out the person who started it all was a Synth.

Edit: Meant to post this as a reply to the other answer, but since it's here I'll say that if you check your settlers after you slaughter them, one of them will have Synth parts. That was the instigator. You can find out if you've been infiltrated by using VATS. Synths have Energy Damage Resistance, although you need a perk to see this. Even though you know it's a Synth, your other settlers don't, so killing the Synth will still make your settlement hostile.