Why did he do it?
Tobias Harris (who seems to have given up sleep during this political crisis) Okumura Jun and Garrett DeOrio have the analyses (I rather like Garrett's sympathy for Ozawa Ichirō's damned-if-you-do / damned-if-you-don't-predicament. It is refreshing.)
Many, many factors play a part in this drama. However, Ozawa must have looked at the numbers for a springtime House of Representatives election; assessed Fukuda's political smarts, looked behind him at his fellow Democrats; looked forward at the political calendar; and decided all by himself that for the good of all he should take an offer of political accommodation if it came his way. When it did, he played the concerned autocrat, going back to the Democratic Party leadership with the offer, awaiting their reluctant rubber stamp.
He had brought them to the brink of owning the LDP, even more so than Ikeda Daisaku could ever dream of owning the LDP (and Ikeda Kaichō can dream mighty big)...
...and dammit, the lazy idealists in the Democratic leadership took one look at the power-sharing offer and turned it down.
"For the people! For democracy!"
To this Ozawa has responded with the exasperation of catcher Crash Davis when pitcher Nuke Laloosh shook off his signs at the top of ninth inning of a two hit shutout.
"Amaterasu Almighty! What more do these guys think they are going to get?"
Having been shown up by the political embarrassments who hired him to do their dirty work, he has quite understandably given up on the ball game.