Chris, BOAC, let's take a wider view rather than a view immediately local to the MLG.
The belly slaps into the ocean. That provides a sharp dowards jolt to the gear. Perhaps tens of milliseconds later the wing becomes involved near the wing roots. There is a slap on the MLG doors mashing them upwards and likely shattering them. So far nothing has touched the MLG itself except a sharp deceleration. Perhaps it has already broken loose from its uplock and started to descend. At some point the MLG doors, whole or partly shattered touch the MLG. The doors are not nearly strong enough to contain the momentum of the MLG. It simply mashes through the doors or forces them (whole or not) out of the way as momentum pivots it to an extended position. Note that about the time the MLG hits the water the large wing surface is going to slap on the water providing a very sharp deceleration to the wing structure. (I'd not be surprised if the spars folded
up in the middle.) The MLG tries to keep going. The pivot point can't. But the truck and tires end can continue so it all pivots open.
I'd be surprised if the MLG was still neatly stowed. It would indicate a severe (too heavy) over design on the uplocks. I would expect to see a lot of torn and bent materials in this area due to these relative forces.
Mechanical Engineering is not my primary field. But visualizing the sequence simply requires a picture of the plane (Wikipoodle has some) and an ability to think in very small steps. Look at it and try to pretend you're seeing, in your mind's eye, that you are watching an extreme slow motion cutaway picture of the events.