It started with a gendered pronouns disagreement, but it lead to Stack Overflow firing a well-liked moderator while lying about her in public.
Apparently mod Monica questioned a new code of conduct in a forum for mods to question such things. Employee Sara Chipps, with the approval of the CTO, fired her and then lied to the press about an observant jew right before Shabbat.
This pissed off a bunch of moderators -- including more than a few who supported the CoC change. The CTO stepped in and made some noises about discussing with Monica, but didn't have the balls to say the truth: Our decision is final, piss off.
Some nontrivial subgroup of mods feel disrespected by the way the company has treated them and the community. They felt, like, for example that they should have the opportunity to comment on a new CoC and to have their opinion seriously considered by Stack Overflow.
In the middle of this is the millions of dollars of free labor donated by mods to run the communities. Approximately 10% of mods have bailed so far. (10 hours per week times 50 weeks per year at a paid rate of $25 or $33-ish to the employer times 70 mods is $1.15m).