If a website is endorsing harassment via images in uploaded chapters, I think it'd be well within the rights for any individual or group targeted this way to want to leave the website, assuming said harassing chapters were reported properly and staff opted to not remove them.
Like, it's one thing to say "group left, so they suck" and another to say "group left because literally another group started trash talking them via chapters uploaded to the same site adding in passive aggressive notes that contribute nothing to the scanlation itself, but I'm still going to say said group sucks for not toughing it out and staying on the site allowing this."
MangaDex's draw is supposed to be that it's actually moderated and meant to be respectful of scanlation groups' choices. If that unfortunately includes allowing people using uploads to bad mouth and bother others to be considered as a choice to "respect" and "allow," then I can understand why groups would choose to let MangaDex know "we've chosen to not allow you to host our stuff alongside theirs."
But maybe that's just because my idea of a tantrum would be to follow suit and start filling their own scanlations with passive aggressive responses sandwiching their releases. So I might be biased.