This is a fun one! And, as with other cases of redundancy, it can be difficult to spot. I'll give you a tip, though, at the end that can help make the task of finding this easier.
First, what's the issue? It seems like a perfectly correct sentence. We use it in speech regularly, so it rolls of the tongue easily.
Sometimes, that's the issue. Our street lingo becomes part of the written word and, after some time, the use of that lingo seems correct.
The definition of the noun "reason" is essentially "a cause or explanation." If the causative nature of a situation is explained by using the word "reason," the use of "because of" becomes redundant.
You'll know what I mean when you see the sentence without it:
The reason he left early was his appointment
Meet the Editor
I'm Coreen, and I am a copy editor, writer, instructor, digital marketer, and student of PR and Communications for organizations doing positive work in the world.