Adjustments - AR

There was a sales order with 2,000 is sales and 5,000 in returns (RMA), this order is in November 2016.  In Receivables, it created an invoice for 2,000 and a credit memo for 5,000.  In January 2017, The system automatically created adjustment of 3,000 debiting AR and credit suspense.  We did not setup a create auto-adjustment.  The question is  Why this transaction in November created an automatic adjustment in January 2017?  What could of trigger this?