This Yomiuri Jan. 27 article is likely as full a rendition of the Tsunoda resignation as Vice President of the Upper House (he submitted his resignation notice to Chikage Ohgi, the president, yesterday) as you will get in a non-Japanese medium.
The Japanese hard copy version carries seven articles (including sidebars), one at the top of page 1, plus an editorial. (The penurious Yomiuri translates only one of its customary two editorials per day, and has decided to go with Prime Minister Abe's policy speech to the Diet.) In case you want to know, Yomiuri thinks the Tsunoda case is more serious than the office expenses brouhaha that has dragged in a large number of LDP names and, among other people, Ichiro Ozawa. Taken in isolation, yes. But the pervasive use of "office expenses" as a bottomless sump for god knows what is a structural problem that further undermines public trust in the political system. The editorial makes no mention of this - what I think is more important - distinction.
The revelation-to-resignation process took so much time because, apparently, nobody in the DPJ had wanted to take charge of the situation. Not that this is not a bipartisan phenomenon, but it's particularly bad when you want to show that you have the potential to run the show if the electorate will only give you a chance.
One of the articles goes into some depth about the DPJ rift in Mr. Tsunoda's Gunma Prefecture between the old school Socialists and the conservatives, with latter coming to take over four of the five DPJ Diets seats in Gunma. Something I wrote about here. According to the article, the takeover led to the discovery of irregularities under the old leadership and their subsequent exposure. The rift, which is echoed, if much more weakly, in the party as a whole, is one very plausible reason for the DPJ leadership's initial reluctance to deal with problem.