According to a report by Daily Sports, Keiichi Hirano decided to test the FA waters because the Hanshin Tigers would not give him a multi-year deal.
"I did not want to say [that I was going to exercise my FA option], but during negotiations, I wonder why they would not [offer me a multi-year deal] even though I worked so hard for them... That feeling just grew stronger. I wanted to play for the Tigers for many years to come," said Hirano.
He continued, "This is not about the money. I also understand the team's desire to use young players and sign free agents. So if that is the case, then I thought this would be my last chance to learn what my current value is on the market. After that, it is retirement, so I decided to exercise my FA option in order to learn about my value."
Hirano then added, "All I have for [Yutaka] Wada, the organization, and the fans, is appreciation. I want to spend more time with the Tigers. I feel like giving back. I also feel like staying, if possible."
Source: Daily Sports 11/12/2012
As for teams that might be interested, Orix and Yokohama have been mentioned the most. Their current interest levels:
Orix Buffaloes (connection: former team)
The Buffaloes are currently focusing all their efforts on signing Tsuyoshi Nishioka. If they fail, then they will most likely look into Hirano as an option. However, it may not be a simple decision for the organization since he is also a class B free agent. Source: Daily Sports 11/12/2012
Yokohama Bay Stars (connection: raise in Kanagawa)
GM Shigeru Takada spoke to reporters on Monday and said that he had no plans to pursue Hirano because the team already has a number of options. Source: Nikkan Sports 11/12/2012
Hanshin Tigers (connection: current team)
The Tigers would like Hirano to return and plan to keep the lines of communication open. Yutaka Wada told reporters on Monday that the team needed him. Source: Daily Sports 11/12/2012, Nikkan Sports 11/12/2012
Incidentally Lotte has also been mentioned.
The Rakuten Eagles may have been considered a possibility, but Senichi Hoshino told reporters they were not interested. Source: Sponichi 11/12/2012