Say we have a sortable table with peoples information in it (First Name, Last Name, Phone, Email, etc.) Initially this table is sorted by Last Name in ascending order (A-Z). Assume in this case we don't want to paginate the table it is just one list, lets say 100 entries.
You try to scroll through and find the name Mike Wazowski. Upon getting about 25% down the list you realize you should have just sorted by Last Name descending so that he'd be put much closer to the top. When you click the little order-by arrows next to Last Name and the order of the rows flips should the table:
Scroll the user back to the top of table
Leave the user scrolled to 25% down the table
My thought is if you reverse the order to have Z-A last names first it's because you want to find a name near the end of the list, so it'd be smart to scroll it back up so they can start near the end. However, if they wanted a name in the P-letter last names and scrolled 40% of the way down and hit descending order they may expect to be very near the P-letters not sent to the beginning in the Z's.