where does negative accounts payable come from

Um... I'm not sure. However, accounts payable is a liability meaning it is money that is going out of the company to that source. So accounts payable is always negative; no idea if thats what you meant by negative accounts payable but hopefully this helped if not post back and i'm sure someone that does know for sure will help ^_^.

It comes from posting checks in payment of bills, without posting the corresponding bill.

Sloppy accounting. Sloppy controls.
Overpayments, double payments, mispostings, etc.
This is also a big red flag for fraud.
Pre-payments should be in an asset account.
You should never issue funds on a vendor account unless a payable has been set up and approved.