This workflow rule fire some time trigger which are currently in pending. for exa: if this field update send email after 30 days or if this record created or no update in last thirty days then send email something like that. Try to find and delete them first. Then you will be able to delete the WF.
Update: To find the WF Queue
When a workflow rule that has time-dependent actions is triggered, use the workflow queue to view pending actions and cancel them if necessary.
To view pending actions:
From Setup, click Monitoring | Time-Based Workflow.
Click Search to view all pending actions for any active workflow rules, or set the filter criteria and click Search to view only the pending actions that match the criteria.
The filter options are:
Workflow Rule Name: The name of the workflow rule.
Object: The object that triggered the workflow rule. Enter the object name in the singular form.
Scheduled Date: The date the pending actions are scheduled to occur.
Create Date: The date the record that triggered the workflow was created.
Created By: The user who created the record that triggered the workflow rule.
Record Name: The name of the record that triggered the workflow rule.
Note : The filter is not case-sensitive.
To cancel pending actions:
Select the box next to the pending actions you want to cancel.
Click Delete.
or check here.