Why would manuscript status change from "awaiting reviewer decisions" to "evaluation requests sent to reviewers"?
This happens frequently if e.g. 


some reviewers agree to review, but don't submit their reports by the deadline (or tell the editor that they won't be able to complete the review after all.) Then the editor tries to find additional reviewers. Or,
the reviewers' reports come in and they are conflicting, so the editor wants more reviews to help him/her make a decision.