This is like saying "when it comes to the 何故," which means you're giving a reason, making this because or the reason is, followed by… whatever you wanna say!

Note that this is a quite formal way to say "because," so you'll mostly encounter this in writing or formal speech, and almost always at the beginning of a sentence in Japanese.

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!