They have many origins, but they primarily originated in the oral lore surrounding the prophet Muhammad. their authenticity and authentication came later, unfortunately, and there was enough time for people to falsify and add. Technically the Quran is a hadeeth as well, but it is a hadeeth that is called mutawaatir or 'successive' because groups of people gathered together to hear it and learn it from the prophet Muhammad (saas), since the beginning. Therefore it has so many separate chains of narrations that is impossible that people agreed upon a lie. There are only a handful of extra-quranic hadeeths that classify as mutawaatir and most of them are partial and vague hadeeths, and some of them have their status called into question. Almost all other hadeeths that we deal with are called ahadie and many of the early scholars agreed that they are based on conjecture and cannot be used or taken for sound, definite knowledge.
As we know many other scholars disagreed and did take them.