My reasoning for the first 2 is that this is hitting both EN and JP, if it was only one branch I could believe a manager being the issue but we know for a fact that the branches are managed separately, so low-tier managers or even branch managers couldn't be making people from multiple branches quit, that leaves upper management, but that is similarly unlikely because those guys do not interact with the talent directly, it would be pretty hard for them to harass people to the point of quitting if they aren't directly messing with them.
So a reason which hits multiple talents in multiple branches must be a company-wide policy change, of which the most likely to affect people is gonna be the revenue split, the streaming hours comes second simply because of the amount of low activity talents dropping. You could argue other shit like perms (which have gotten better not worse) or project cancellation but unless that got extremely bad I cannot see it setting off so many people at once so quickly.
My third idea is the mixed bag explanation, in which there were a handful of unhappy talents that all got inspired or pushed to quit by each other, in this case the true reasoning for quitting could be many things, from health issues, to project cancellation to burnout to wanting to advance their career. Which is honestly the most likely to me. I have begun believing that Aqua in particular broke something within the internal culture of the company, maybe people were afraid to leave before because of peer pressure or they thought they wouldn't succeed without Holo, and her leaving was just the push many of these girls needed to finally pull the trigger.