When the cold blows in, you want a beer to warm you up. That's why winter seasonals typically feature a higher ABV% along with being more robust and malty.
After the chilly season, you're ready to transition into a lighter tasting beer. That's why brewers tend to lighten the malt notes to transition you through the spring.
When the sun is shining, you want max refreshment. That's why summer seasonals are typically lighter in color and flavor with notes of fruit and/or spices.
As the leaves float down, you're ready for something a little more hearty. That's why brewers bring back the malty ales and marzens along with incorporating flavors like pumpkin.