Thanks but the news is not so clear regarding exactly how he quit.
You say he "asked to quit" but I understood that as meaning he "was asked to quit" - by the board probably !
It would make sense that he was asked to as there was a petition delivered to them.
Anyway, all nuances at this stage. Come on Mark - say something to the people!
You might not get a definitive answer (because there isn't one probably), but why don't you head over to the BTC Foundation forum and read the discussion there for yourself.
The foundation forum is open to the public now, and there's more than one thread discussing Mark's/mtgox's removal from the board, and whether he should be forced or "asked" to leave the board.
I suspect he saw that eventually he would be removed from the board forcefully, so he stepped down "voluntarily" while he still had a chance.