Automatic approval typically occurs when there is no approver. For example, you say "Chuck's Manager needs to approve this" and I have no manager defined. The workflow says "Rather than be stuck here with no way out, I'm going right on through." Check the underlying data to ensure it meets the conditions of your workflow. If you have cases, like mine, where there is no approver, check before you enter the approval and reroute to a notification, a waiting for, etc, or check after the approval and adjust the workflow accordingly.

Reference:

Workflow Concepts - ServiceNow Wiki

Creating a Workflow - ServiceNow Wiki

Using Workflow Activities - ServiceNow Wiki