Why “Lose” Evidence?

The latest twist in the IRS saga is that the IRS may have deliberately destroyed evidence. I'd like to know why they would do that. What were they afraid of? The IRS apparently came up with their substitute for laws declared unconstitutional in the Citizens United decision but those laws had the support of the Democratic base and even some moderates. Coming clean might increase Republican turnout but it would also increase Democratic turnout. They didn't have to worry about about the Justice Department. Even if the Democrats lost an election, the President could have arranged last-minute pardons. Impeachment and conviction was unlikely as the Democrats controlled the Senate. The IRS might lose a court case or two but the Democrats had a history of ignoring courts. (This is being revived.)

The only possibility that makes sense is that they're covering up activities that even leftists find sleazy. I am not imaginative enough to know what that is. Even the theory that the President's re-election campaign used IRS data seems inadequate.