Microwave popcorn odor

Why does commercial microwave popcorn always seem to have a strong, rancid odor when popping?  I don't detect such when I microwave just popcorn, but all bagged popcorn always seems to have a characteristic, nauseating odor.  About the only thing worse is microwaved fish.
 
Thinking it might be due to artificial ingredients, I recently tried a Newman's Own style that claimed to contain natural butter with other natural flavors (whatever that means) with no mention of artificial flavorings, but noticed no improvement, so that's likely not the cause.
 
Are there any bagged brands that don't result in that typical obnoxious odor? At home, I normally pop just jarred popcorn (either on the stove or in a microwave), but would like to have a bagged brand to utilize when traveling (pop in motel rooms).