George Dvorsky details five possible scientific explanations for Westeros’ seasons of unpredictable length. A “wobbly planetary tilt” is one possible reason:
In the episode “The Kingsroad,” we learn that Westeros has at least one moon. It’s very possible, therefore, that they have a very small or distant moon, that is causing a variable tilt in their planet’s rotational axis.
It’s interesting to note that, according to legend, Westeros used to have two moons, but “one wandered too close to the sun and it cracked from the heat” pouring out a thousand thousand dragons. Well, dragons aside, it’s conceivable that some kind of cataclysmic celestial event could have wiped out their second moon, which would have thrown their planet’s rotational axis out of whack.