Definitely, the fact sam altman could tell investors not to invest in competitors like xAI and anthropic says a lot

Because OpenAI cannot offer them competitive compensation, anymore. Not when they can court VCs and get hundreds of millions of seed funding to setup their own AI startup. Also OpenAI is by this point a mature project so lots of people might feel a bit stifled in their freedom and they might be dying to explore new ideas.

This was just published the other day and goes in-depth about your question:

https://fortune.com/2024/10/01/openai-sam-altman-mira-murati...

IMHO, as an outsider enthusiast:

It's an grab bag of more-or-less common reasons that are present in different relative amounts in different people. In rough order of importance:

1. People are concerned that the leadership is not acting with the best interests of the company in mind, specifically Sam and Greg. See: https://www.reddit.com/r/OpenAI/comments/1804u5y/former_open... and other coverage from the ouster scandal from last winter. This even lead to him walking back from billions of dollars, which is pretty remarkable: https://www.cnbc.com/2024/09/26/openais-sam-altman-tells-emp...

2. People are concerned that the leadership is not acting with the best interests of the public in mind, specifically in regard to the risk of recklessly creating a meaningfully "human-like" AI and giving it control of significant resources. Here's a discussion of how they've moved away from their very pro-public Charter: https://arstechnica.com/information-technology/2024/09/opena... and here's an opinion piece on the risks: https://time.com/6266923/ai-eliezer-yudkowsky-open-letter-no...

3. Other firms are offering huge incentives to OpenAI employees, and engineers are doing what engineers always do: job-hopping to jump a rank or two in the job ladder. From last year, but still relevant IMO: https://techcrunch.com/2023/11/20/openai-ai-talent-poaching-...

4. People are concerned that OpenAI's future is not as bright as it seems given its current (objective) technical dominance, a discussion centering around terms like "moat" and "propriety data" and "open source vs. closed source". Some challenges facing the company include: quite uncertain lawsuits of existential size (https://www.nytimes.com/2023/12/27/business/media/new-york-t...), the fact they're losing $5B/y without a proven path out of that (https://www.datacenterdynamics.com/en/news/openai-training-a...), and, on a meta level, your question itself -- they've lost basically all of their top scientists.

For good general coverage and the quintessential recent example, look for commentary on Mira Murati's exit last week, e.g. https://www.cnn.com/2024/09/25/tech/openai-technology-chief-...

Probably because Anthropic/Google offered double their current pay with less work.

Are many people leaving? Or is it a couple of very visible people?

If you come for the king, you best not miss.

Partially it's weirdness from Open AI transitioning from non profit to profit, partially it's from people who are delusional about the dangers of AI, partly it's great unheard of offers from other companies.