Is there a core of fairly widespread (not fringe) theory which argues for another factor as the origin of social classes?
Oh goodness yes!
During the later ages of classical theory, social classes were often thought to be divinely ordained. Each person was born into a certain position in life according to God's will. This view was shared by many different bodies of theories at the time.
For primary texts, you might consider Epictetus (Discourses and Fragments). Other early Christian writers share this view, but his works tend to be short and fairly easy to read.
That early Christian theory was largely built on the works of Greek philosophers. According to the Socratic school, social classes were embodiments of our reasoning abilities. Plato divided the soul of humanity into three parts: reason, spirit, and desire. Each part corresponds to a social class.
For example, rulers in Plato's republic were required to be philosophers (capable of abstract analytical thought and capable of discerning what is true). They embody the "reason" part of the soul. On the other hand, workers embody the "desire" part - they want many things and will impress their wants on society.
If you want a primary text, this is explicitly articulated in Plato's Republic. There are elements of it present in Socrates, Aristotle, and related writers. For a summary this may be a good start.
Many liberal philosophers (especially anyone in the vaguely Kantian realm) identified two social classes based on their degree of independence. Civil society is the class that is driven by dependence: people who work in order to fulfill their own needs.
A good ruling class is independent of these needs (they don't have to work in order to fulfill their needs). This allows them to be objective utility maximizers: since they don't have any personal interests that are influenced by their political decisions, they are free to make political decisions that are in the best interest of society.
Further reading: There are no good references or easily-read works by Kant. Sorry.
In the 19th century we saw the rise of social darwinism. Social darwinists believed that notions from evolutionary theory could be applied to human society. One idea was that social progress was an indicator of how well adapted a person (or family, culture, etc.) was to their environment. Members of wealthy classes were more fit; members of the working class were less fit. The strata naturally developed as an equilibrium between competing social interests.
For more info, read the works of Herbert Spencer.
A common thread in anarchism is that social imbalance (including class) is a product of the state. A direct example is that states create prisons to administer punishments for breaking the law, which creates a criminal class. A less direct example is that laws outlining property rights require people to work in order to "earn" a place to live, food, etc. This creates an impoverished social class.
This thought continues abstractly in much of the post-structural theorists. Foucalt, for example, focuses on power relationships which are abstractions of this same thing. In this sense, we can see the state-based power relationship in society all around us.
Further reading: Political Justice (Godwin).