My non-lawyerly answer to the OPs question is:
You, even as a US citizen entitled entry to the USA, are required to present yourself at a port of entry to an Immigration officer. That immigration officer is allowed to verify your right to entry with questions if they deem it. Once you have legally immigrated, you are then required to present yourself and your belongings to a customs officer to ensure that you are not bringing anything illegal into the country, as well as pay a legal duty on anything over the standard exemptions that you are importing.
Why the officer asked you why you were coming back to your own country does seem silly. Who knows? Maybe they just wanted to make friendly conversation, or maybe they are trying to catch you in a lie. Don't take this the wrong way, but people do try to illegally enter the US, probably on a daily basis, for a variety of reasons. The mere possession of a US passport does not mean that it is not a forgery or you are the person the passport belongs to.