If you check your POA-Table, you might find that it is getting big. Searching for the problem, you might also find out, that each of your record is shared with the owner of the parent record.

 

This blog-post describes implicit sharing really well.
Update: The Blog seems to be deleted.

To get ridge of this, you can use the organization settings, to disable the feature. Use the managed Solution named “Dynamics CRM Organization Settings Editor” to set this information in a comfort way. You can download the solution from this link. https://orgdborgsettings.codeplex.com/

 

The option you are looking for is the one with the name “”. Set this to true and to be sure, execute an iisreset.