Why does my I485 goes back to review after the interview?

It is common for USCIS to review the file after the initial interview, because they want to make sure that nothing was overlooked before approving the file. The fact that you were separated during the interview is an indication that the adjudicating officer has concerns about the bona fides of your relationship. They can approve the application as is or they can schedule subsequent interviews until they have enough evidence to make a decision. In situations where they suspect marriage fraud, they will mail you a Notice of Intent to Deny and give you up to 30 days to submit additional evidence to overcome their suspicions.

How do you know that it is going back if your just had your interview today? The fact that you were interviewed separately may mean that the USCIS has questions or concerns about the bona fide nature of your marriage and that may be what they are going to review.

If the officer said there is a background check issue, then normally there is. Such an issue can take a few days, a few months, or even a few years.