On the category pages for my site, there is a Sort By dropdown control at the upper-right of the products listing. Why does it sometimes show options for sorting by highest or lowest price?

On a category page, there is a Sort By option that lets the user change how the products are sorted on the listing. The standard options are: Default, Default: Reverse, Alphabetically, and Alphabetically: Reverse.

If any of the products in the category have a Teaser Price enabled in the settings for that product, then the Sort By dropdown list will also have options for Price: Low to High and Price: High to Low.
If a Teaser Price is set on any of the products in that category, then you will see the additional options on the dropdown menu. There will also be a tag on the product image showing the teaser price as the Starts At value.