IRS letter stated, We had to do some additional processing on your tax return not able to honor your request for direct deposit of your tax refund, sending check. Why?

IRS letter stated, We had to do some additional processing on your tax return. Therefore, we weren't able to honor your request for direct deposit of your tax refund and are sending you a check instead. As a result, you should receive a refund check in the mail in 3-4 weeks. 


What reason would cause this?