Some of these may have been artificial and built by native peoples. Others might be remnants of ancient tsunami and hurricanes, changing sea levels, floods and so on.
The same phenomenon occurs elsewhere. The Netherlands have tiny features they call hills. In East Anglia, the flattest region of England, there are tiny hills. If you look at any pictures of the Gog Magog Hills south of Cambridge, you could walk up them barely breaking a sweat. In fact in some of the pictures, you'd struggle to spot the hills. In Scotland, they wouldn't even be considered a hill range.
No one believes me, but there is a respectable example in Norwich called Kett's Hill. It's not huge, but it has enough of a slope on it to qualify as a hill, and I've walked up it.