This has been common this year.
During the review process, they look for math errors on your return (extremely rare in TurboTax) and they also check if you owe back taxes, unpaid child support, or other debts.
The IRS started processing returns later than in prior years and as such has a backlog to work through. They also just recently started processing returns claiming EIC and/or ACTC.
If they need to make any corrections, they may
offset (reduce) your refund. In some cases, the correction might even increase your refund.
Once that part's done, the government approves your refund, which means it's ready to be deposited or sent. At that point the IRS site will update with a direct deposit date.