Causes of Bell's palsy. Bell’s palsy occurs when the nerve that controls the facial muscles (facial nerve) becomes inflamed or compressed. It's not known what causes the facial nerve to become inflamed, although it's thought that a virus, possibly a herpes virus, may be responsible.auses of Bell's palsy. Bell’s palsy occurs when the nerve that controls the facial muscles (facial nerve) becomes inflamed or compressed. It's not known what causes the facial nerve to become inflamed, although it's thought that a virus, possibly a herpes virus, may be responsible.