Hey Hannes,
The way the Suppier (Vendor) and Custom MDG works is it always creates the Business Partner first. Depending on your version of MDG it might be slightly different, but the CL_MDG_BS_ECC_ACCESS is always called in order to save the BP.
Once BP is saved, based on your CVI configuration, a Supplier or Customer would be created copying big chunk of the BP and enhancing it with the specific data from the Change Request.
Hope that explains why the class is always called 
Regards,
Boris