According to tranfermarkt.de he's been out with an ankle joint injury this whole year and was projected to come back at the end of this month, at which point there would be only 4 matches left to play. And he isn't the youngest anymore. Unfortunate course of events it seems. This is why it's always good to extend contracts early or automatically.
Should sign back with Kawasaki =) will close once official http://www.hochi.co.jp/soccer/world/20150621-OHT1T50205.html
because it's nice to be able to get back to your former club instead of another one, it's not always possible. I don't think Yusuke would have had trouble finding another team in 2.Bundesliga as a free agent, if he wanted. He's always had positive seasons. I think he just wanted to get back home and I don't see any problem, he's also 29.