They were just so angry at the fact that she was happy and im sure they got scared that she might look better in the ball, so when your angry at someone you'll give it your best to hurt them.

But it could be the fabric...

I think they're very masculine. Just look at the size of those feet!

Think how great of quality a dress would be i fi was made by mice.

Because they were very angry!

The scene's told from Cinderella's point of view. She sees this mess and chaos of ripping where her stepsisters are acting like cruel feral beasts. The exaggeration here is to help the viewers empathise with how much this is emotionally affecting Cinders.

I think it was due to the fact that the sisters were very angry and if they destroyed her dress she won't be going to the ball. Also the main portion of the dress was old. It was made from a dress that Cinderella's mother had/wore. It was only made up and embellished to make it into a more modern dress for Cinderella.

They had an adrenaline rush.