No-one knows for sure what happened, they have started trying to raise it today by pumping it out (not much luck so far), but as you would expect the marina is full of theories!
Its a fairly old cruiser stern, the general consensus is that it had been taking on water for a while and then reached whatever entry point in the hull (large engine vents perhaps?), but at the same time nobody noticed, which is strange as there are a lot of liveaboards at this marina.
Its a very big marina, incidentally, and there are plenty of staff around but as I say no-one noticed it, staff or other moorers. I have heard of a case at another marina where staff actively didn't intervene in a similar situation due to 'health and safety' concerns, but I can't imagine that would be the case here, plus most of the staff are also boaters so could empathise at how horrible this situation would be.