I'm not sure how accurate they are but commentators on yachting etc channels are saying the yacht sank at the stern first, then onto it's side.
That suggests hatches at the rear may not have been closed properly.
These are not, as many posters on here have assumed, roof window type hatches for air; they are large hatches for areas of storage for items you might float or drive off the yacht etc. There might also be one for access to lower levels.
They apparently have sealed doors/openings inside but commentators say it's very common for the requirement to seal then to be overridden for convenience.
There is also talk of how the keel should probably have been fully down to counteract wind etc. from the side, but that it seems it was not (due to that not being standard when the yacht was not sailing, and due to them perhaps wanting to minimise the snap back rocking/rolling effect of the yacht in a storm).
Even with the rolling it would have been doing, if the hatches weren't sealed/closed, water could have gotten in any of the three (I think) at the rear.
There is also an issue raised re. the main doors which apparently have the fault of opening when the yacht heels relatively steeply. This is apparently common on these type of doors. They can be fixed closed (dogged) but it seems like that wasn't done. (And unsurprisingly since people were coming and going etc). That would only have hampered escape if anyone was in that space anyway.
This is probably going to be a case of human error, a small amount of yacht design (especially if the manufacturer's operating instructions didn't specify to lower the keel in severe conditions, even at anchor) and severe/freak weather. But it's all still speculation.