Sounds like he just decided it was time. He wrote a pretty gracious post about it.

https://hindenburgresearch.com/gratitude/

Funny, I wrote this without seeing what the full URL was (I’m on my phone) so the use of the word “gracious” preceded me knowing the path was /gratitude

To me, the Hindenburg Research announcement seemed to suggest that they were being threatened by the subjects of their research.