Since it is the IRS that does the reviews, they are the only ones who could tell you why.
It is likely to be a routine review of Earned Income Credit, Child Tax Credit or the American Opportunity Credit. Tax returns with large refunds may be subject to extra scrutiny to prevent fraud.
This doesn't mean you are being audited, just that refund will take a little longer.
If they need additional information from you, they will contact you by sending a letter.