There is no explanation for the erratic seasons. GRRM (just like any storyteller) creates mystery and interest in his world and fills in the details that are required, and not so many of those that are not.

This is something he is very aware of. He will tell you that like any intriguing medieval world map, his world map also has tall tales, mysteries and stories about foreign countries, big monsters and weird men, lining the edge of the map.