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.