I think there are two aspects to this, the first one as you mention is that often reviewers just asked to cite relevant papers (and it's not unusual they come from the same author, because the fields are small).
The other aspect is, that there are a some black sheep who will ask you to cite a whole bunch of barely relevant papers (often >5) all from the same group. With those you very clearly see that they are abusing the system. I suspect that these instances cloud people's judgement on the first aspect, so they always interpret the question to cite as fraudulent.
As a general advice if you suspect reviewers are try to push their citation counts up, a note to the editor (or PC if a conference) will typically help, because most don't look very favourably at this.
The other aspect is, that there are a some black sheep who will ask you to cite a whole bunch of barely relevant papers (often >5) all from the same group. With those you very clearly see that they are abusing the system. I suspect that these instances cloud people's judgement on the first aspect, so they always interpret the question to cite as fraudulent.
As a general advice if you suspect reviewers are try to push their citation counts up, a note to the editor (or PC if a conference) will typically help, because most don't look very favourably at this.