Spoiler - Cause of girl's problem

This pretty much sums it up. I would like to add some points:
Anneliese's parents and the two priests were not the only people who believed in her possession. The first person who considered her possessed was a homemaker named Thea Hein. She regularly organized pilgrimages to San Damiano (apparition site not approved by the Catholic Church) which were also attended by Anneliese and her family. Mrs. Hein claimed Anneliese exuded a horrible smell of burning coal, like the devil, that she tried to rip a so-called "miraculous medal" (google, if you don't know what it is) off her neck. She was also present during the exorcism sessions and it is claimed that during a very intense moment, Anneliese hurled the heavyset Mrs. Hein around.
So even in the real story, there were two women who were linked to her "problems", to put it mildly. Her mother was a devout Catholic, albeit a "fallen" one. She had given birth to an illegitimate daughter before marrying Anneliese's father, and she had to marry him with a black veil. According to local gossip, the child was fathered by a priest. Inspite, or maybe because of her sin, the purity of her daughter was her top priority. Pants were fashionable in the 70s, especially bell-botton jeans. Anneliese wanted to wear them, but her mother said it was against God's law. Anneliese's grandmother was very Catholic, too, and she dragged Anneliese as a young child to early Mass, every day, like the local peasant woman Barbara Weigand, who was considered a saint by many around Anneliese.
So Anneliese lived in the shadow of many devout Catholic women, and she was supposed to become one of them.
In the movie, the issue is reduced to Michaela's relationship with her mother.
Anneliese Michel, that's how I feel, had a love-hate relationship with what could be considered Catholic pietism. She strived to be obedient. Modest skirts instead of jeans. A fiance with the same Catholic upbringing instead of free love. Praying the rosary instead of going to parties. She also did not question the more controversial beliefs held by her family, like visiting non-approved apparition sites. Anneliese's family, the Heins, the two priests and others belonging to the same circle where very much against receiving communion into the hand, receiving communion standing, nuns not praying the rosary, priests wearing street clothes. Most of the exorcism dialogues, recorded on tape by Mrs. Hein, are about these things.
Her family home was cluttered with religious pictures, statues, crucifixes, and of course, rosaries. When Anneliese had one of her fits, she often destroyed these items. In her normal life, she devoutly used them.
My impression is that Anneliese was deeply disturbed by growing up both in the permissive 70s and a circle of Roman Catholics having trouble dealing with Vatican II. She was taken advantage of by traditionalist priests and misunderstood by a handful of less than smart but more than devout people.