One of the more interesting aspects of “Game of Thrones,” the third season of the TV adaptation of which starts on March 31, 2013 on HBO, is the variation of the seasons. Summers can last years. Winter can last a lifetime. Indeed, the phrase “winter is coming” takes on a dire connotation indeed when one can be snowed in for decades.
While it is dangerous to imagine a scientific explanation for things in a world where dragons exist and magic works, a March 29, 2013 article in Space.com makes the attempt.
The way seasons work on Earth results from the fact that the planet has a slight tilt. When the northern hemisphere tilts toward the sun, summer occurs there. When the northern hemisphere tilts away from the sun, winter occurs there. It is the exact opposite in the southern hemisphere so that when it is summer in North America and Europe it is winter in Australia, South Africa, and most of South America and visa versa.
However some plants wobble in unpredictable ways, making the seasons variable in length. On the world where the land of Westeros lays, perhaps the northern hemisphere points toward its sun for years causing a long summer and then wobbles so that it points away from the sun causing a life time long winter.
Another possibility is that the star system where the Westeros planet resides has several other planets that effect that world’s orbit, pulling it away from its sun for a while and then pulling it toward its sun as various, unpredictable times.