Possible a very stupid question but.... Why/how are Demons lords suddenly appearing in the Underdark? 

One of my players asked me and honestly I couldn’t answer. I’ve read through the book (and maybe I’ve just missed it) but how are Demons lords suddenly showing up here? I know some are being summoned, but how the hell does a Kuo-Toa suddenly have the power to summon the prince of demons???


I’m really enjoying running the campaign and my players are having lots of fun so it’s not going to ruin my enjoyment. I’m just wondering if I missed something like a weakening of the barrier between the planes or some sinister force at work. 

Apologies if I’m missing something super obvious