Some of your users accounts go into negative.
The most common reason of an account goes into negative is when the “play and pay” payment method is selected.
The second reason is when the credit limit is not set in the “pay and play” method. You can set the credit limit under Usergroups Additional Tab
A guide discribing how to use the different payment modes and setting them up can be found here:
Another reason for accounts going into negative can be the “credit limit” feature: