Q: What does a status change from "under review" to "reviewers assigned" mean?

The status “reviewers assigned” indicates that the reviewers accepted the invitation to review your paper. Thus the subsequent status “under review” means that the manuscript has been sent out for review. However, the change of status back to “reviewers assigned” indicates that at least one of the original reviewers might have declined the invitation to review after going through your manuscript in detail. This could possibly be because the reviewer(s) may have realized a potential conflict of interest. Another reason might be that the reviewer(s) may have realized that they do not possess the expertise required to evaluate some parts of your manuscript. This is quite a common situation and you need not worry. You should expect the status of your manuscript to change back to "under review" once the requisite number of reviewers have accepted the invitation.

Related reading:

• Why does the status change back to "Reviewers assigned" after 3 days of "Under review"?  
• What does a change in status from "under review" to "reviewer assigned" mean for a revised manuscript?