An author may become a hybrid author for a number of reasons.
For example, in traditional publishing, a book contract normally contains a clause that allows the rights to an out-of-print book to revert to the author. Many authors who were previously traditionally published are now recovering the rights to their out-of-print books and independently publishing new editions.
Another example would be an author who is being traditionally published, but chooses to independently publish a new book, either because it isn't the type of book the publisher normally handles, or the author wants to maintain complete control, or the book is too time-sensitive for the timeline of traditional publishing.
A final example would be an author who starts their career by publishing independently, then accepts a contract with a traditional publisher for later books or other editions (such as print editions or translations).