ELI5: Why do some packages expand in the freezer?

Why do some packages in the freezer expand?

I sometimes find packages I've put in my freezer have inflated. I haven't seen it in meats, or cooked vegetables. Most recently a sealed package of ground coffee was significantly inflated, and didn't collapse upon defrosting.

What's going on here? Do some foods outgas when cold? Shouldn't the cold make things contract rather than expand?