More info at https://retractionwatch.com/2023/10/09/how-thousands-of-invi... and https://arxiv.org/abs/2310.02192 . As the latter makes clear, this type of fraud is most likely done by journal editors (or their assistants), not by the authors:

> When registering a new publication and its references at Crossref, a publisher may sneak extra undue references in the metadata sent in addition to the ones originally present. Then, digital libraries (e.g., SpringerLink) and bibliometric platforms (e.g., Dimensions) harvest these metadata, undue citations included. These sneaked references are processed and counted even if they are not present in the original publication.

The three journals in this particular case are all published by Technoscience Academy, an OA publisher operating out of India (not one of the well-known ones). I would think twice as an author before I submitted to any journal from this publisher, lest my paper is abused for manipulations like this (although I'm not sure if it has any journals worth submitting to anyway).

NB (because I got confused first): This is not really about Hindawi. Hindawi published the (trash) article that these fake citations were pumping up, but the pumping-up happened using Technoscience Academy journals.