Outliner just reversed alphabetical order

Why did my outliner just reverse the alphabetical order (Z to A instead of A to Z)?

How can I set it back?

You probably clicked the header. Click it one more time to switch between descending and ascending ordering. The little arrow that shows next to Item Label indicates which ordering is currently set.
OutlinerSort

That was exactly what I was looking for (googled, too, but missed it) .

Thanks so much!