If the car is parked outside (and has alot of exposure to the elements) and the headlights are cleaned too often that will happen.
You can try getting some Meguiar's Plastic Polish. Some have said it did wonders for their lights. I have been fortunate to have a garage at home and covered parking at work, so my lights have not yellowed at all....I also give them a good cleaning every time the car is washed.
And welcome to AF
