Other mods are leaving as a sign of protest for recent and well-known facts.
By now, mods resignations appear to be just ordinary business and common users have probably started to get used to it, but since their departure doesn’t seem to really worry SE I wonder if mods as a category were actually a target (or one of the targets) of recent changes in SE.
Were mods a sort of lobby within SE? Did their (supposed) power ever disturb or represent a “menace” to SE? That would explain much of what we have seen so far in terms of disruption of the old order on SE sites.