When water gets squeezed between a ship’s hull and a sand floor, it speeds up. As water flow speeds up, its pressure drops, pulling the hull down to fill the vacuum. The effect is more pronounced at the stern, and so the ship settles into a squat: bow up, stern down.
Yet another article that needlessly complicated things by invoking the Bernoulli principle. It’s a lot simpler to explain: the space behind the stern needs to suck water into it so the stern area is at lower pressure, while the space around the bow needs to push water out of the way so it’s at higher pressure. The closer you are to the sea floor or bank, the bigger the effect since there isn’t much space to push water away and pull water from.