Reader BA thinks it’s about more than Super Tuesday:

It’s a good point that releasing his returns after Super Tuesday is a good strategic move for Romney’s quest for the nomination, but I prefer another theory (I think Krugman’s) that this allows him to release his returns filed this year and claim that “I’ve released my returns!” That allows him to have his people put together a tax return that’s as uncontroversial as possible, as opposed to releasing the returns he filed last year, when he probably wasn’t thinking about eventually having to release them.

It’s worth noting that George Romney, Mitt Romney’s father, released 12 years of his tax returns when he ran for president in 1968. More recently, John Kerry released 10 years of tax returns and Barack Obama released seven years of tax returns. But the norm is different on the Republican side: John McCain released two years of tax returns, and George W. Bush only released one.