Take a look at the sentence again:
When Matt shaved his head, his mother worried that he had joined a
cult the real reason for the bald head,
however, was that Matt could get more attention and sympathy from girls
who thought he was sick with a dread disease.
The problem occurs between cult and the. His mother worried that he had joined a cult is a complete sentence. The real reason for the bald head ... begins the next complete sentence. Because these two parts have only blank space between them, they make a fused sentence, a major sentence error.
To fix the problem, you could put a period after cult and then capitalize the T for the.