Freezing affects food in different ways depending on how it is done. If FAST FREEZING, small ice crystals are made that may ensure the preservation of texture of food when thawing is done. In SLOW FREEZING, large crystals form which distort texture of foods. In the case of vegetables, either of the two processes will present some level of damage to the products, with slow freeing yielding the most damage in quality.
Freezing does not stop chemical or enzymatic reactions, it only slows them down. So these may still progress depending on the state of vegetables, e.g. whether they were blanched or not in the case of enzymatic reactions.