Does customs know how long you were out of the country? I thought only immigration had that information in front of them, but they don't see how much checked luggage you have. Or does immigration write something about the length of your trip on the bottom of the customs form?

In any case, I'd guess that you were just randomly selected for some additional screening. IME, both immigration and customs will often act like you have something to be nervous about just to see if it will provoke a reaction. I don't think that act necessarily has any basis in fact.

Mrs. hockeyguy and I (different last names) have even been challenged as to whether we're actually married -- seemed like a weird thing to challenge us on as I'm not aware of any particular benefit we would get from pretending to be married. Plus, I'm sure immigration knows full well that we live at the same address and they can easily see that we've traveled together for years. With all that, I have to assume it was just a "probing" technique to see how we would react. We've never had 15 minutes of screening, but a couple times they've taken maybe 3-4 minutes to ask some extra questions.

In short, you probably didn't do anything to trigger the extra search, and even if you did, it's probably not the luggage (and I doubt I&C will tell you what it was). I'd chalk it up to bad luck and I wouldn't worry about it for the future.