When someone asks you 何故, or "why," you did something, you respond with, "何故なら blah blah blah." That makes it mean because. Why did you do that? Because I was mad! Why did you dance? Because I was happy!
This reading is an exception but you learned them already because they're the same as the word 何故, which you learned a few levels ago. It just has some kana attached to it!