How can I fail quests if I haven't started them yet, or even accepted them?

For example I went to prison to save the new sheriff for Primm, and while killing the Powder Gangers I got the message that I have failed the quest "I fought the law". Same thing happened again while cleaning Vault 3, I killed a fiend named Motor Runner while helping NCR and same thing happened, I failed some other quest apparently related to him. Last example, I went to Freeside and entered the laser gun store (can't remember the name now) and all of sudden everyone started shooting at me! Also I failed some quest.

Can anyone explain me why this happens? I can't access Gamefaqs, so I can't read articles in there.