OK, the problem is that your T3 framework is automatically applying the "Chosen" jQuery plugin to all dropdowns on your page, turning them from plain HTML dropdowns into the enhanced Chosen style ones, with the additional features (like search). My copy of T3 / Purity III doesn't do that, it may be an option somewhere in the template settings.
And the problem with that is, the Chosen dropdowns don't correctly fire JS events. So, for example, a Fabrik CDD element that "watches" a database join dropdown for a 'change' event, so it knows to fire off the AJAX call to rebuild the CDD options, never gets that 'change' event, because the altered dropdown isn't firing it.
We do have code to handle that, but we only run it if Fabrik itself knows that it is changing a dropdown to be "enhanced" with Chosen.
So ... if you go to the Fabrik global options, under the Form tab, and set "Enhanced Dropdowns" to "Yes", Fabrik should then know to add the code we need to fix the issue with the Chosen dropdowns, and fire the correct JS events.
-- hugh