What is an unstable coalition?

The recent resignation of Yukio Hatoyama as Prime Minister of Japan made me curious as to the tenure of the Japanese government. The following table is from Wikipedia. Except for Junichiro Koizumi, the average length of office has been less than 2 years. Is it a coincidence that a large part of the change in government is also during what is considered Japan's lost decade?

				Days in	Years in	Months in
Prime Minister	Took Office	Left Office	Party affiliation	Office	Office	Office
Noboru Takeshita	11/06/87	06/03/89	Liberal Democratic	575	1.57	19
Sōsuke Uno	06/03/89	08/10/89	Liberal Democratic	68	0.19	2
Toshiki Kaifu	08/10/89	11/05/91	Liberal Democratic	817	2.24	27
Kiichi Miyazawa	11/05/91	08/09/93	Liberal Democratic	643	1.76	21
Morihiro Hosokawa	08/09/93	04/28/94	Japan New	262	0.72	8
Tsutomu Hata	04/28/94	06/30/94	Renewal	63	0.17	2
Tomiichi Murayama	06/30/94	01/11/96	Socialist	560	1.53	19
Ryūtarō Hashimoto	01/11/96	07/30/98	Liberal Democratic	931	2.55	30
Keizō Obuchi	07/30/98	04/05/00	Liberal Democratic	615	1.68	21
Yoshirō Mori	04/05/00	04/26/01	Liberal Democratic	386	1.06	12
Junichirō Koizumi	04/26/01	09/26/06	Liberal Democratic	1979	5.42	65
Shinzō Abe	09/26/06	09/26/07	Liberal Democratic	365	1	12
Yasuo Fukuda	09/26/07	09/24/08	Liberal Democratic	364	1	12
Taro Aso	09/24/08	09/16/09	Liberal Democratic	357	0.98	12
Yukio Hatoyama	09/16/09	06/01/10	Democratic	258	0.71	9
			Average	549.53	1.5	18.07
			Median	386	1.06	12
			25th percentile	309.5	0.85	10.5
			75th percentile	629	1.72	21
			Min	63	0.17	2
			Max	1979	5.42	65