Okay, I don't think her resignation has that much to do with paparazzi shots. Tama-chan mentioned years ago when Bubka came out with a picture of her kissing some dude with an afro and an article about him "orally stimulating her clitoris." Seriously. Maybe, if she was already unhappy, something like that could've pushed her over the edge. But I seriously doubt that it was just this one thing.
In my opinion, this was a long time coming. The difference, in age alone, between Mari and the other girls in the group is a big deal. But this is not some snap decision. I think Mari is too smart for that. There is either something going on that we don't know about and may or may not ever hear about at all or this was a decision that she's been considering for a very long time. Maybe both.
Someone joked that she was pregnant on JPM and that was deemed inappropriate. But something like that really wouldn't surprise me in this situation. Who else has left H!P as suddenly as this? Rinne, Michiyo, Rika Ishii and maybe those Sheki-dol girls are the only ones I can think of. But this is Mari. You'd think they'd at least try to capitalize on her leaving with a big grad concert and some other crap. Maybe she's not pregnant or something that's such a big deal but I don't think that this is just some temper tantrum over a tabloid.