Q: Why does the status change from "Awaiting reviewer assignment" to "Reviewers contacted"?

The status "Reviewers contacted" means that the editor has sent out review invitations to the requisite number of reviewers (usually 2 or 3). If all the reviewers accept the invitations, the peer review process will start and the status will change to "Under review." However, if one or more reviewers decline the invitation (because they have a conflict of interest or are busy with other engagements), the status will change back to "Awaiting reviewer assignment" and the editor will have to look for more reviewers. It is nice that your journal's status descriptions are so detailed: it is much easier to understand what's going on with your manuscript at the backend. 

Related reading:

• Why does my manuscript's status keep changing from "awaiting reviewer selection" to "awaiting reviewer assignment"?
• What does a status change from "Awaiting reviewer assignment" to "Reviewers Invited" mean?