How to customize Category on new tickets

We have a field “Category” that is automatically being filled in for many of our tickets. Specifically many tickets that are automated notifications that are set to close. But also occurs on regular helpdesk tickets. They are receiving a category that does not belong to the tickets. 

The Trigger listed below does not have any action regarding to Category, how can I determine how the field Category is set so we can get rid of so many tickets that incorrectly have a “sales” category?