There's a few ways this could go and very likely no video feed to be sure. It had the look of a large volume of water quickly released. Massive tonnages break

There's a few ways this could go and very likely no video feed to be sure. It had the look of a large volume of water quickly released. Massive tonnages breaking off of a glacier is still going to be mostly ice on impact .. there's a chance debris and ice chunks made a temporary dam as the bulk of the now shattered glacier ice released water putting pressure on that weak dam - leading to collapse and release.