Whenever I take over an enemy capital that was a Holy City for another religion, I always convert it to my religion and remove the other religion from it completely.
Sometimes I've noticed that after a few turns, inexplicably, the original religion will spontaneously gain 1 follower, which is enough to give it 10 pressure even though it's not a majority religion.
What causes this? I always make sure to also clear all surrounding cities of the original religion (at least I make sure that my religion is the majority in all cities, meaning that the other religion shouldn't spread). Is it caused by having even one religious follower of the old religion somewhere?