It's a special kunyomi reading, called a jukujikun reading. Jukujikun readings are when a native Japanese word gets assigned to multiple kanji as a whole, rather than to a single kanji (which is what typical kun readings are).
Common examples are abundant. 今日 (きょう), 明日 (あした), 一日 (ついたち), 大人 (おとな), etc.
There are many, many more.
So, think of these the same as you would any kun reading, in terms of their origin, but because of how they are assigned to multiple kanji as a whole, you can't split them apart and say "this part goes with this kanji, and this one goes with this other kanji."
Awesome! Thanks for the info, every bit helps : )
Why いかが?
I was just curious as to the origin of this pronunciation, seeing as it doesn't seem to be an onyomi nor a kunyomi reading of the kanji