I don't see it as mystery as why it happened. Perhaps you are thinking there was some specific trigger for his stepping down. I don't think so. When the language becomes as popular as Python has, the head of the project is going to become a target for a lot of unwanted attention. The "assignment expression" PEP was a good example of that but not the sole cause.
Even if infuriating jerks are 0.1% of the population, when your language has hundreds of thousands of users, you are going to deal with a lot of jerks.
Frankly, I'm surprised he lasted as long as he did without going mad or something. I would not wish for any of my friends to be subjected to that kind of attention.
We were very lucky to have him leading the project for so long. Python will survive without a BDFL and I hope he enjoys the vacation.
I'm not exactly implying that it's a mystery. Mystery or not it I think it warrants some public consideration. If the reason indeed was 0.1% jerks, that should be confirmed and addressed in writing.
Even if infuriating jerks are 0.1% of the population, when your language has hundreds of thousands of users, you are going to deal with a lot of jerks. Frankly, I'm surprised he lasted as long as he did without going mad or something. I would not wish for any of my friends to be subjected to that kind of attention.
We were very lucky to have him leading the project for so long. Python will survive without a BDFL and I hope he enjoys the vacation.