There used to be a field watcher that would have been helpful here, but it's missing since the Next Experience UI. Have you ruled out Business Rules? Is there a workflow running on this Catalog Item? Does the RITM Priority update like this with every Catalog Item? If they were not very good consultants they could have done this in an onSubmit Catalog Client Script or a Client Script on the sc_task table.
If you are in a situation where you could give me access to a non-prod instance or screen share I could probably sniff it out fairly quickly.
