Why would manuscript status change from "awaiting reviewer decisions" to "evaluation requests sent to reviewers"? I submitted a manuscript two weeks ago to a journal. I was following the progress of handling my manuscript depending on the status revealed by the journal's website within my account.
The status history showed the following:

1. Manuscript accepted for processing
2. Evaluation requests sent to reviewers
3. Review is pending
4. Awaiting reviewers decision
5. Evaluation requests sent to reviewers


The journal never informed me about the decision. What are the reasons behind this behavior? Is this normal ?
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.