Sales Order reverts from Pending Fulfillment to Pending Approval (?)

This one has me baffled.

We receive a number of sales order via our eBay integration. These orders are approved by our customer service team (document status goes from 'Pending Approval' to 'Pending Fulfillment').  Once we have confirmed the payment has been received via PayPal, this sales order is then edited and the payment information is updated, etc.  What's weird is that the document status then reverts from 'Pending Fulfillment' back to 'Pending Approval'.  I've been trying to reproduce this behavior on our sandbox, and cannot get it to happen.

So the question I have is this - what are the potential circumstances that would cause the document status of a sales order to revert like this? I've watched this happen on our production environment, and have gone through the system notes line by line and applied the same changes to a test order, without getting this to happen. There isn't an Item Fulfillment or Invoice record when this occurs, so I don't think this is the root problem.  There is something else going on, but I just can put my finger on it.