Don't you remember what happened when Achan was unfaithful and took some of the things that belonged to God? This made God angry with the entire nation. Achan died because he sinned, but he also caused the death of many others.
The LORD had said that everything in Jericho belonged to him. But Achan from the Judah tribe took some of the things from Jericho for himself. And so the LORD was angry with the Israelites, because one of them had disobeyed him.