It's dubbed the "BK Pumpkin", and it features a dual-slit bun that is supposed to resemble a pumpkin. According to Excite, it's also stuffed with slices of kabocha, which is the traditional, green Japanese squash. For ¥100, you can add five times the number of kabocha slices for a grand total of ten. Burger King is officially calling this burger "Bomb Pumpkin" ("Bakudan Pumpkin" or 爆弾パンプキン), and is running a special campaign until November 11.
"Bomb" (bakudan) is sometimes used in Japanese fast food for items that are packed with stuff.
Kabocha is a traditional fall and winter dish in Japan. Even though the burger will be available until December, by using "pumpkin", Burger King is most likely evoking Halloween, trying to ski in the event's wake. By calling the larger version the "Bomb Pumpkin", however, Burger King Japan is inadvertently evoking World War II.
Google searching "bomb pumpkin" (爆弾パンプキン) in Japanese pulls up countless images of pumpkin bombs, which were conventional aerial bombs developed by the Manhattan Project and dropped on Japan. They were sometimes painted pumpkin orange and resembled the Fat Man that dropped on Nagasaki.
The Japanese wiki page for pumpkin bomb even sits atop the Burger King pumpkin burger stories.
Times change, and most Japanese probably won't make the connection. Still, this is probably not the imagery Burger King wanted to evoke, and it's somewhat vexing that, you know, nobody seems to have bothered to check Google.