You probably enabled the setting unintentionally on your profile.
Go to your personal profile, click on
Look and Layout preference, then you check
Show most recent posts at the top. It should be ticked, reason why threads appear bottom to top on your account.
Just unmark it to return to descending view (I think it should be descending order when it's arranged from earlier replies to later ones).
Ensure you click on
change profile after to save the changes.