Why does ice form mostly on one side of frozen sliced bread?
Your freezer causes this, as your experiment has shown.

Freezers don't magically cool down their contents, like ovens have a heat source, freezers have a "cold source", so in your case the back cools / freezes first, encouraging condensation and subsequently the formation of ice crystals on the colder parts. If your freezer has an auto-defrost cycle, this may contribute as well.