How would they know? Did the demons tell them? Do they have supernatural knowledge.
This kind of talk gives religion a bad reputation. Show me the evidence. Show me documented studies.
Simply read the books written by official Catholic exorcists. One common denominator in demonic harassment, obsession and possession is dabbling with the occult; and they specifically warn against the Ouija board, which is the conjuring up of spirits, something the Sacred Scriptures explicitly forbids.
When one makes a habit of living in mortal sin or dabbles int the occult, one opens themselves up to the demonic, simply because you are putting yourself near the wicked spirits.
When mortal sin and evil activities are allowed to be present in one´s soul unrepented and masked by rationalizations, they begin to fester and to develop into all kinds of strange psychological complexes that darken the mind and most often develop into mental problems.
A good analogy is not cleaning your home; soon you have waste and filth attracting all kinds of strange visitors—insects and cockroaches that reproduce and make a home there with you. Untreated it turns into an insalubrious infestation that is very hard to get rid of, carrying all kinds of disease that attack the health of the body. It´s the same thing with the spiritual soul. Mortal sin and dabbling with the occult, which itself is a mortal sin, makes one weak and vulnerable, being that one has put themselves in the devil’s territory— like getting close to a chained wolf that now can bite you.
After casting out a demon, Jesus Himself warned about the unclean spirits that can make their home in your soul:
“When a strong man, fully armed, guards his own house, his possessions are safe. But when someone stronger attacks and overpowers him, he takes away the armor in which the man trusted and divides up his plunder. . . . “When an impure spirit comes out of a person, it goes through arid places seeking rest and does not find it. Then it says, ‘I will return to the house I left.’ When it arrives, it finds the house swept clean and put in order. Then it goes and takes seven other spirits more wicked than itself, and they go in and live there. And the final condition of that person is worse than the first.” Luke 11:21-26