Q: What could be the reason if the status changes from 'Under Review' to 'Reviewers Assigned'?

This ‘reverse’ status change – from ‘Under Review’ back to ‘Reviewers Assigned’ – is not uncommon. It usually happens for two reasons. One, that one or more of the reviewers has backed out from the review either due to unavailability or a conflict of interest, due to which the associate editor (AE) of the journal needs to look for and assign reviewers again. Two, the peer reviewers may have different opinions on the manuscript, due to which the AE sometimes has to send the manuscript for review again – to a third reviewer.

In your case, as the status changed to ‘Reviewers Assigned’ after a month, the latter seems to have happened. For more information, you may refer to this previous query by another user: What does a status change from "Required reviews completed" back to "Under review" mean?

To answer your other query, ‘Under Review,’ as the nomenclature suggests, means that the document is being reviewed. In most cases, it means that it is being peer reviewed. In some cases though, it refers to the internal (journal) review done before the peer review. You may learn more about the status through these previous queries by other users:

• Does "Under Review" mean that the paper has passed the editorial check?
• Does "Under Review" mean the paper has been assigned to an external reviewer?

As this status has just been updated, you could wait for some time, say, about two or three weeks, for the next update. If the status doesn’t change by the end of the month, you could send a mail to the editor requesting an update. Note that as you said this is a Springer journal, the process might take a bit longer, due to the huge volume of manuscripts that are typically submitted to a Springer Nature journal.

All the best for the next steps.