I have spent hours today looking at my 23andme DNA matches, and while I have not checked every single one, it seems to me that ALL of them descend from ancestors of my paternal grandmother. I am 100% sure of about 3 of them, and so far every person in the list matches someone (that matches someone..etc..) that shares DNA with these 3. I am not aware of any pedigree collapse. I know the birthplaces of all but one of my 32 3xgreat-grandparents. All are very distinct geographically, between grandparents, except for a handful born in "Ireland".
I already know that these particular ancestors had many children etc, and that in my far-from-complete Visual Phasing exercise I have a particularly low amount of DNA from my father's father. However I am not satisfied that this explains why all 1500 23andme matches are linked to one grandparent.
This phenomenon does not occur on other DNA sites holding my DNA (but they have many more matches).
None of them have only one tiny segment, lowest match of the 1500 has one seg of 16cM.
Could it be some kind of mistake on 23andme's part?