Unrelated "terror bombing"?
I found this information on the Stellaris reddit which got it from the wiki. (For some reason I cannot load the wiki myself)
When you are performing orbital bombardment successfully killing a POP on that planet can trigger the Terror Bombing modifier if your empire is not militaristic or fanatic militaristic. This lowers your happiness by 5% if your empire is pacifist or 10% if you are fanatic pacifist on planets in your empire. The only way to get rid of this debuff is to stop killing POPs through orbital bombardment for around 3,600 days.