There are multiple reasons this could happen:
1. Awaiting appraisal from the bank
2, Awaiting BPO or computer appraisal
3. Awaiting short sale approval.
Here's an excellent similar question with details:
In the meantime, the agent wants to keep their options open. Typically in the agent's remarks on the listing (not visible to the public), the agent will update the status periodically, e.g. it will say "1 offer accepted, two backups in hand. Awaiting BPO."