The order of entries in a rollup field matches the order of the records in the corresponding linked record field. By default, new linked records are added to the end of the list, thus linked records tend to be in chronological order.
I sell an automation script that sorts linked records. one of the most common use cases is maintaining a reverse chronological sort for linked records. Note that this script only sorts linked records as new records are added, it does not sort existing linked records. I also have an app for sorting existing linked records that is available for purchase but requires a custom install. (If you are interested in this app, contact me privately.)