Fracture on implant supported cantilever bridge: any thoughts?

The first PA shows the bridge 3 days before the rest of it fractured.  I'm not sure why the cantilever was used here instead of an implant in #21 site as there is ample bone.

Source: Dental Implants Discussed by Experts - August 18, 2017 Category: Dentistry Authors: osseonews Tags: Clinical Cases Surgical Source Type: news