There are always types of events that are not suitable for public discourse (pretty much any form of harassment or abuse, where victims are still subject to pressure or were yet unable to process what happened falls into this category). I have no insight into what happened but it's not hard for me to imagine what could prompt moderation team to resign w/o disclosing specific instances.
Right. It may also be possible that there is some hope of this action restoring normality. i.e. as a result of this protest, the Core Team become more accountable. At that point, I assume the specific incident(s) may be dealt with according to the Code of Conduct, which may or may not involve transparency. Either way, it could be premature and possibly prejudicial to air those now.