Not voter fraud, but offshore accounts
Well, I'm not sure why he would want so badly to vote in Massachusetts unless one of his kids was running for something and he really wanted to be able to vote for him. It'd have to be a pretty local office for one vote to matter very much.
I still think the reason is because of the offshore account amnesty the IRS offered in 2009 where those who had offshore accounts they hadn't been paying taxes on could pay the back taxes they owe (and some penalties I think) but would not be prosecuted, if they declared them within the amnesty window. This would have shown up on 2009 tax returns, and would be pretty embarassing for a guy running for president.
This seems the most likely, as it accounts for why he would only release 2010 and 2011, even though when he was vetted by the McCain campaign as a possible VP pick in 2008 he gave them 23 years worth. He'd probably be happy to make public those 23 years, but if he took the amnesty the 2009 return would be a campaign killer for him if it became public and it could be proven it was his genuine return.
Even if someone did steal his return Romney may have requested that PWC say they found no evidence of a hack even if they did so he can try to claim that the returns eventually released are fakes. If they really do have something they'll release it at some point - they are just seeing if the Romney campaign is dumb enough to pay $1 million in untraceable hush money before they release it anyway.
This is really the only thing that makes sense for why Romney is willing to take the heat for not releasing more years of taxes, especially when his own father set the standard for modern campaigns by releasing 12 years of returns. Any amount of heat is better than what would happen if it were shown he was illegally dodging taxes via offshore accounts. Worse yet, this would have still been going on during his previous presidential run in 2008! Even if he somehow still got elected he'd have lost all credibility to speak about any type of tax reform.
It is kind of funny this happened, as I had been speculating to a friend about this a couple months ago and I told him that there are probably too many people who have had access to his returns that may have stashed away a copy, either at his accountant, at any banks he's given them to, or even within the IRS itself. There are some people so partisan they'd be happy to go to jail for a few years if it insured "their guy" got re-elected. I didn't expect a hacker, but I guess I shouldn't be surprised.