The most likely cause of this is lack of an outline or organization. While freewriting , without an outline in mind, it is easy to get off track. Because ideas are not properly organized it is easy for a student to jumble ideas. In the middle of one thought, a freewriter may jump to another, causing a fragment. Imitation of speech patterns in writing Writing as we speak is probably the greatest source of sentence fragments.
Humans have a tendency to use fragments when conversing casually.
A writer must understand that written prose cannot imitate speech patterns. While we might think in incomplete sentences, they cannot appear in a composition. Learn to turn incomplete thoughts into complete sentences. Because a writer knows what he or she is trying to say in his or her paper, it is easy for him or her to look over sentence fragments when checking for errors. If fragments are found, determine what is wrong and either take out what shouldn't be there or add what is missing.