It is important to be careful to not define activities that deviate from normal human behavior as mental illness. We should not expect average human behavior to represent a ceiling on virtue, wisdom, or rationality.
Granted I could have dug further. To my defense I just echoed a quote in the article, which states he held a resentment that "eventually metamorphosed into a paranoia which is evident in the pages of Récoltes et Semailles. - Excerpt, "The IHÉS at Forty" by Allyn Jackson (1999)".
I did however do some digging in order to produce this response to your comment, and I found an interesting article about Grothendieck on Psychology Today [0]. It also mentions an apparent trend among some mathematical geniuses to develop mental illness, and it expresses hope that a connection may be found thanks to "advances in the nascent field of creativity-psychopathology neuroscience", of which I know nothing. But I stick to my original speculation: that overworking might be part of it (until I'm convinced otherwise).
As for Grothendieck the author has this to say about the later stages of his life: "He believed himself to be in communication with Plato and Descartes, and even with God himself. The belief in signal transmission is a signature psychotic delusion".
[0] https://www.psychologytoday.com/us/articles/201707/the-mad-g...
Thanks for finding those details. I didn't know that Grothendieck was hearing voices.