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?

[larry-strickler]

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?

[Critter]

From the IRS :   

I requested a direct deposit refund. Why are you mailing it to me as a paper check?

There are three possible reasons. They are as follows:

• We can only deposit refunds electronically into accounts in your own name, your spouse's name or in a joint account.
• A financial institution may reject a direct deposit.
• We can’t deposit more than three electronic refunds into a single financial account.

https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions