Okay, you made me do some research. A natural occurring Ponzi is a term that was coined by Noble prize-winning economist Robert Shiller. Here is the definition of this kind of scheme that can be found in his book called "Irrational Exuberance":

“Ponzi schemes do arise from time to time without the contrivance of a fraudulent manager. Even if there is no manipulator fabricating false stories and deliberately deceiving investors in the aggregate stock market, tales about the market are everywhere. When prices go up a number of times, investors are rewarded sequentially by price movements in these markets just as they are in Ponzi schemes. There are still many people (indeed, the stock brokerage and mutual fund industries as a whole) who benefit from telling stories that suggest that the markets will go up further. There is no reason for these stories to be fraudulent; they need to only emphasize the positive news and give less emphasis to the negative.”

This term was also mentioned by World Bank that criticized and attacked Bitcoin back in 2014: https://www.coindesk.com/markets/2014/07/17/world-bank-report-bitcoin-is-a-naturally-occurring-ponzi/

As far as I understand, "a natural-occurring Ponzi scheme" is another way of saying "a bubble driven by natural laws of supply and demand", which is basically correct because bitcoin has always been the product of a free market. The problem with the word "Ponzi" is that it implies malicious activities of some kind and sounds very negative. Natural occurring malicious activity sounds no better than a Ponzi scheme. I think that this term is mostly used by those who want to attack bitcoin and make others into incorrect thinking. It is misleading and dangerous, and doesn't help much to explain things.