Q: What does a status change from "Awaiting reviewer score" back to "Awaiting reviewer assignment" mean?

The status "Awaiting reviewer assignment" indicates that the editor has sent out reviewer invitations. Once the requisite number of reviewers (generally 2 to 3) accept the invitation, the status changes to "Awaiting reviewer scores" which is the equivalent of "Under review." However, in your case, the status changed back to "Awaiting reviewer assignment." This is probably because one of the reviewers declined the reviewer invitation after accepting it initially. 

This is not an uncommon situation. Sometimes, after going through the manuscript, reviewers realize that they are not equipped to review the manuscript or that they have a potential conflict of interest. In such cases, they inform the editor immediately and the editor begins to look for a new reviewer. Once the new reviewer accepts the invitation, the status will change to "Awaiting reviewer scores" once again.

Related reading:

• Why has the status changed from "Reviews completed" to "Under review" and then to "Reviewers assigned"?