This seems highly plausible, either 1) He did disclose his Swiss bank account to the IRS prior to 2009 which would be indicated by his prior returns, in which case releasing these returns would put this all to bed or 2) He did not disclose this account in which case it would not show up on his returns, and releasing them would prove he had acted illegally, almost assuredly dooming his campaign. That fact that he refuses to release any of these past returns leads me to believe it is the second case, otherwise the political pounding he's currently taking would not be worth it.