If one of the forward windows blew in, against the force of the water on it........?
The other thought is that with the nose so low under water, the angle between the centre of the rotor and the pivot point (usually the C of G but now the nose) is very different and might have tripped the heli up. Rather like pushing a wheelbarrow with all the weight at the front and then hitting a large stone with the tyre.
Than god that I failed the Bristow helicopter course. We don't do that sort of flying in a 767!