From Matt Yglesias, an intriguing theory.  In 2009:

Wealthy U.S. taxpayers, concerned about an Internal Revenue Service crackdown on the use of secret overseas bank accounts as tax havens, are rushing to meet a Thursday deadline to disclose those accounts or face possible criminal prosecution.
The concern was triggered this summer when Switzerland’s largest bank, caught up in an international tax evasion dispute, said it would disclose the names of more than 4,000 of its U.S. account holders.

Yglesias adds:

Romney might well have thought in 2007 and 2008 that there was nothing to fear about a non-disclosed offshore account he’d set up years earlier precisely because it wasn’t disclosed. But then came the settlement and the rush of non-disclosers to apply for the amnesty. Failing to apply for the amnesty and then getting charged by the IRS would have been both financially and politically disastrous. So amnesty it was. But even though the amnesty would eliminate any legal or financial liability for past acts, it would hardly eliminate political liability.

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