By default GetNextWork prioritize assignments based on their urgency. But in this scenario, need to prioritize based on pxCreateDateTime. Customize the list view - GetNextWork in Assign-Worklist/WorkBasket class in your ruleset to introduce your own business logic. Also modify the listview sort order on pxcreateDatetime so that the eligible cases will be sorted and the first created case will be available.
Note: we consider the case creation date time not the assignment creation date time.