A nascent trend on YouTube is to take contemporary dramas and imagine what their 1995-style opening credits sequences might look like. The first one appears to be this Walking Dead one, followed by Breaking Bad, which is the best of the bunch:
The seasons in George RR Martin's medieval fantasy are a random, unpredictable mess. They could last anywhere from a few months to a decade and there's no way to forecast them. As the story opens, the characters are near the end of a long, ten-year summer. They also worry about the coming winter, which will cause mass starvation if it also lasts years on end. This wonky climate is an irreplaceable part of Game of Thrones. Westeros would not be remotely the same without it.
But grapevines have a life cycle that depends on regular seasons. In winter, grapevines are dormant. Come spring they sprout leaves. As summer begins, they flower and tiny little grapes appear. Throughout the summer the grapes fill up with water, sugar and acid. The grapes are finally ready for picking in early autumn, then go back to sleep in winter. This cycle is why wineries can rely on a yearly grape yield. Obviously, in Westeros, something must be different about how grapes work.
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.