I would bet that you have the module Pathauto enabled. It allows you to create path aliases for mach of your site stuff like nodes and taxonomies. Go to Configuration > Search and metadata > URL aliases > Patterns. That's where you enter the alias patterns for content, taxonomies, etc. Set the default alias to [term:vocabulary]/[term:name]
It should be mentioned for any case that the basic Drupal path is [term:vocabulary]/[[term:name]/%
