Filter/Sort Post Form On Taxonomy page

That happens because wp_dropdown_categories, by default, defines the name of the dropdowns as ‘cat’.

You are showing 2 dropdowns in the form, and both are named ‘cat’, because you didn’t define a name for them. So when you submit the form, you get two query vars named cat.

For example, if you want the second dropdown to be called states, you would have something like this:

<?php wp_dropdown_categories('name=states&show_option_all=All U.S. States&taxonomy=states&depth=2&orderby=name&show_count=1'); ?>

But how do you plan to use these dropdowns in filtering the content?