Since he won't release his tax returns, and does admit they'd be politically damaging, all we can do is guess.
This is one in a series of speculative diaries wondering what could possibly be worth the price he is paying by remaining secretive.
Today's idea: Ann Romney would be shocked and dismayed to see some of Mitt's business dealings. Perhaps she has been kept out of the loop and had to sign those multi-hundred page tax returns on faith. Maybe she'll find that Mitt invested money in places she finds immoral or took deductions that she knows aren't legitimate.
Mitt doesn't want to have the trouble at home.