I can say it doesn't have to do with questing for a mysterious artifact, or water chip, or a GECK. :D

For writing dialogue, we have a style guide which all the designers must follow or be severely punished.
A few of the rules include things like having all threatening, hostile responses come second to last, and the goodbye response is always the last response.
Each designer is responsible for writing the dialogue for his own areas.
In general, we try to make the responses neutral enough that any type of PC could potentially say it. I found it annoying in Arcanum when my brutal half-orc barbarian from the desert wastes was forced to say things like "Pardon me, good sir".
We also throw in flavor-responses to respond to the player's choice of skills as well. A super-mechanic might be able to say something like "I notice you've got the new G-58 generators installed" where a non-mechanic player could only say something like "What are those things over there?"
[...]I'm not sure how it makes it too easy. It's just a convenient way for the player to end dialogue and start hostilities. By having the hostile response ALWAYS second-to-last, it prevents the player from accidentally starting a fight.

Keep in mind that the above is a way for the player to deliberately make the NPC angry. The player can still say the wrong thing to an NPC and start a fight. If the player is talking to the mysterious person and says something like "Why, yes, it was I who defeated those scummy bandits" and the mysterious guy happens to the sole surviving member of the bandit camp, then the next thing the player sees from the stranger will likely be "I shall kill you and avenge my brothers!"