They want 2008 Tax Returns. They suspect (and they may be right, and the may have inside info on it) that Romney participated in the Swiss Bank Amnesty program. Mitt is probably hoping by releasing this years, it’ll please most people. I think most people could care less if he did the Swiss Bank Amnesty, but the Dims/Media would hammer him over the head with it relentlessly and would push everything else (jobs, budget, obamacare) off the table for the rest of the election.
They made the same request of Hillary.