Moses's brother is a smooth talker. How do we know? Well, God appoints him as the official communicator in Moses's dealings with Pharaoh (4:14-16). Sounds like a good system, right? One guy does the magic, the other guy does the schmoozing. And we're even told that the team works well together (4:30-31). Apparently things are going swimmingly… until they hit the desert.
In the nice, big priestly interlude (25:1-31:18), stuff goes down. Before, Aaron was just Moses's right-hand man. Now there's this whole set of rules to make sure that he and his heirs are properly cared for and tutored in the arts of ram-slaughtering.
Think about it. We went from, "Hey, Moses. Here's this cool dude who will help you out in your quest" to "And you shall give Aaron 8.23172283 ounces of ram's blood every year and make sure you hold the tartar sauce." What gives?
Well, scholars suspect that a priest may have written and assembled parts of Exodus. Aaron is the forefather of the priests, so these are his descendants writing. You can imagine that they'd want to make Aaron look good. And boy to they have some work to do. Remember, Aaron messes up big time in the story. He allows the Israelites to build the golden calf, then lies about it, indirectly causing the deaths of 3,000 people. Oops. As if it weren't clear enough, the text then squarely blames Aaron for everything: "Then the Lord sent a plague on the people, because they made the calf—the one that Aaron made." (32:35). Yikes.
So the priests have to preserve the original story while making sure that Aaron comes out looking good. The huge priestly sections—that comprise about ten chapters in total—were a great place to start. How do you think they did?