This is getting so annoying but I have a theory. These companies are basically a season ahead of themselves because they don't want to get beat to the punch. A company that relies heavily on seasonals does not want to have the previous season's beer sell out then they have very little on the shelf. Then larger breweries see these other companies putting out the next seasons beer and feel that they have to get THEIR beer out to compete. Wash-rinse-repeat.
However it works its annoying as hell because it forces people to hoard beers for the upcoming season when they are actually supposed to be enjoyed. I do this with Oktoberfest every year because it's starting to run out when I actually want to drink it. Same with the really good pumpkin ales, they'll start hitting shelves in August which is ridiculous but if I don't get what I want, I won't have any in October.
when I was around 22 at a party and ended up trying to fight a billy goat, swearing I was re-enacting a the camel punch scene from Conan