Occasionally, if the estimated taxes section gets started either on purpose or accidentally but not finished - the program may remove your withholdings from the calculation (because it expects you to go through the section and finish it and update the withholdings for 2019).  This often leads to estimated tax vouchers being created when you have a refund.  The other scenario is when your refund is being created by refundable tax credits such as the earned income credit or American opportunity credit.

If you are receiving a refund and you expect a similar situation next year, you can safely ignore those vouchers.