Whenever your character speaks with an NPC you invariably insult them. It is completely accidental. In fact, it is completely impossible for you to insult something purposefully.

Mechanics:

Whenever you speak to an NPC, you roll the d20. If you roll 8 or lower, you insult the NPC, resulting in -2 charisma whenever speaking to this NPC. This effect can stack.

However, if you roll above 18, you insult the NPC, but they enjoy the insult (Ask your DM if they want to regulate that, which would complicate things but also probably be more interesting. Regulation meaning they would decide the roll based on the personality of that particular NPC. I suppose they could do that with the above effect as well, like if they wanted it to be above or under 8. Maybe they could also control the -2 charisma effect as well.) and find it hilarious, which is a permanent +5 charisma when speaking with this particular NPC.

Also, you can roll a d8 once per combat to use vicious mockery. If you roll above 4, you can use vicious mockery once. If you roll under 4, you cannot use vicious mockery, and, if your roll under 2, you cannot use vicious mockery again until the next rest. When you cast vicious mockery in this method, the verbal component is ignored completely, meaning you can still cast when unable to speak or make noise. I'll say what's happening is that people are just insulted by your presence in general here, when you cast Vicious mockery without a verbal component.