The phrase is "Bull in a china shop." Picture a giant creature weighing a ton. Picture this store with all these dainty little porcelain objects. Picture the shopkeeper as she is about to see thousands in inventory about to be shattered.

It means that Cassie is very clumsy like a bull in a china shop.

A bull could get into a china shop if it had escaped from a nearby farm or field. Similarly, if the china shop had left a door open then any type of animal could get in.

the phrase is ' as potentially destructive as a bull....'

The expression is usually "a bull in a china shop" as we see it. This might just be a variation on the theme.