On the back of my bag of frozen broccoli florets it emphatically warns me to not refreeze defrosted vegetables. Why is that? What is wrong with refreezing vegetables after leaving them defrosted for a couple of hours?

I couldn't figure it out through a Google search. Thanks a bunch :)
[![The broccoli bag](https://i.stack.imgur.com/ItZYI.jpg)](https://i.stack.imgur.com/ItZYI.jpg)