The whales died (another one died as well) and a museum brought them on shore to cut them open and have their skeletons removed so they could be displayed. The scientist who cut the whale open knew that its insides would be under pressure but he wasn't sure how much.
When the whale died the bacteria inside it began to eat it from the inside out, producing gas and reproducing more bacteria. As more and more bacteria began to reproduce, exponentially more gas began to accumulate inside the whale. With some natural external factors accelerating the reproduction and gas producing rates of the the bacteria and the whale's insides providing the perfect environment for this bacteria to feast upon (and thus shit a lot more which means more gas is created), a shit ton of pressure built up within the whale causing this beautiful video to exist.