End of July, to be precise, on or shortly before the 27th, I saw the about me of the user in question saying "delete me." This is part of the self-deletion process of SE for established accounts. The account was then deleted on July 27th.
User accounts of the same user on other sites were not deleted and continue to be active to this day. Indeed a math.se account linked to them was recreated at some point. (It's still a 101 account, though. I do not link it on purpose. But whoever is so inclined should have no major trouble to find it.)
Thus, it seems overwhelmingly likely to me that the user asked for their math SE account to be deleted.
I observed on at least two occasions that the user was asked in chat about their reasons for this deletion. I did not see any reply, and thus assume they have no interest to discuss their motivation for this decision.
I hesitated to answer this question, as I am not sure about its suitability, but there it is the information I can offer as an attentive observer, in the hope of doing more good than harm.