Received a LTR 2645C letter advising IRS needs 60-days to review. What is the cause for the delay?

[Bsch4477]

Only the IRS can tell you that. 

[xmasbaby0]

TurboTax does not get any information from the IRS after you file your tax return, so no one at TT knows that the IRS sent you a letter or why the IRS is reviewing your return.   If the IRS wants more information from you, they will request it from you by sending a letter.  Meanwhile, all you can do is wait for the IRS.  Only the IRS can give you any information.