Michael,
I think grass bunkers were in the great minority years ago.
My feeling is that many sand bunkers were allowed to become grass bunkers over time for a variety of reasons, some financial, others personal agendas.
Today, I believe you see them introduced more frequently by architects.
At a club I'm fairly familiar with Robert Von Hagge had a unique concept that I liked, but, the membership found it too difficult to cope with.
He had several truncated, cone shaped grass bunkers, open at one end
with the floor of the bunker mowed to fairway height and balance of the bunker at rough height. The floors of the bunkers were well below the level of the putting surface, which was elevated, but, the floor remained at about fairway grade.
Eventually, the grass on the floors was allowed to grow to rough height, making extraction a simple process.
The right, large greenside bunker on the 17th at GCGC was misguidedly converted into three sand and one grass bunker.
Why ? Because someone thought it would be better than the expansive bunker that guarded the entire right side and that having sand bunkers AND a grass bunker would add diversity in terms of features. It was a bad idea.
Yet, the disfiguration has been allowed to remain.
These two unrelated, evolutionary histories convinces me that many, if not most grass bunkers evolved from either sand bunkers, hollows or excavated areas.
I'm not a big fan of them, especially with the advent of the LOB Wedge.