One of the things that has been going on lately has been a rise in religious extremism. When that happens, a lot of people leave.
The average person doesn't want to be associated with a terrorist, a racist, etc etc, unless they actually are one also. Heavier religious extremism will cause more average people of that religion to look for another, or just quit following the religion. Where that's impossible, they just tend to pay lip service without actually worshipping. Going through the motions, as it were.