A video report from the Washington Post attributes Japanese anger over Hatoyama's betrayal of Okinawans as the reason for his fall:
Japanese citizens were promised by Prime Minister Yukio Hatoyama that Japan would remove an American Marine base from their island. After reversing stance and deciding against removing the military base on Okinawa, the public hit the streets to express their rage. His betrayal split his ruling coalition and helped trigger his resignation.However, Foreign Policy in Focus editor John Feffer points a finger at U.S. government interference for Hatoyama's lack of choice in the matter:
Yukio Hatoyama’s political career is dead, and Washington’s fingerprints are all over the murder weapon.Read the rest of this spot-on analysis here.