I have been on the editorial board of a middling journal (IF ~4) for some years now. I get sent a lot of the manuscripts that are in my sub-sub-field. My job is to obtain reviews and make a decision as to the fate of each of these manuscripts. It's become apparent to me that there is a group of reviewers who all display the same phenotype when it comes to their reviews. They all i) are quick to agree to review manuscripts in our common sub-sub-field, ii) submit their reviews on time, and iii) will recommend acceptance or minor revisions for all manuscripts. All.
This journal rejects ~70% of all submissions.* ~40% are desk rejects so ~50% of all manuscripts sent out for review are rejected.
Did I mention that this bloc of reviewers are all strongly linked to one particular well-known member of our sub-sub-field? Former trainees, co-authors etc. Given that pretty much none of the manuscripts they've reviewed for me in the past have authors from within the group, I doubt this is a organized ring of shady reviewers. In fact, having interacted with some of them I suspect this is more a misguided** attempt to raise the profile of the sub-sub-field promoted by Dr. Well-Known.
I don't use these reviewers anymore.
* Yes, there really is that much crap being submitted even to middling journals.
** "Misguided" is somewhat euphemistic.