The storefront page is titled "replacement-parts-shop." After clicking on a category, the breadcrumbs seem to alter a bit in contrast to the storefront page.
Now, the breadcrumb that corresponds to the storefront is not called "replacement-parts-shop" but rather "replacement-parts" which is not what it should be. If you click the "replacement-parts" breadcrumb, it takes you to a page completely different than the storefront page.
The name changes because the menu item of your storefront is called "replacement-parts-shop" while the menu item that you set in the left module's "menu" option is called "replacement-parts".
So change the name and alias of your replacement-parts menu item and that will help. Note however, that you can't have two menus with the same alias. The name can be the same but not the alias as it is used by Joomla's SEF system to know what is the current menu.
That aslo explains why you don't go back on the storefront when on click on that breadcrumb link since these are two different menus...
