Why exactly do leafy vegetables turn to mush when frozen uncoooked? I've heard 2 explanations: because of enzymes that get destroyed when cooking (which is chemical) and because plant cells expand when the water inside freezes, and then the cells burst (which is mechanical). Which one is it really? Or both?

On that note, why does cooking (blanching/sautéing) preserve their texture after freezing and thawing? The fact that it does seems to be an argument against the 'burst cells' explanation, since the cells are still full of water after freezing.
In freezing, it is mostly the bursting of the cell walls as the ice crystals expand.

Much like a water balloon holds its shape when it is intact, but becomes flaccid when punctured, so to the cell walls comprising the vegetable.

See related, for why a similar thing happens when you cook spinach:
Why does spinach lose its texture when cooked?

Cooked vegetables are already soft and their metaphorical balloons partially deflated.  It is not going to get a lot worse for the freezing.  The par cooking is mostly disable enzymes which would change the color of the vegetable to dull olive drab, or negatively affect the flavor.  

See: Why blanche vegetables before freezing?