Many reasons that truck may be on the lot. I'm guessing its because the original owner found it wasn't big enough for their needs. I have a close friend that bought a gas F-250 knowing he was going to be buying a fifth wheel. Less than a year later he traded it on a F-350 diesel dually AFTER he towed his new fifth wheel once.
I also have a cousin who bought an HD Silverado because it was his idea of a status vehicle, even though he sat behind a desk and had no trailer to tow. He traded it at less than 20k miles once he got sick of paying for fuel and found out the ride wasn't as luxurious as other status vehicles.
I wouldn't be scared of it. Unless it had major accident damage, I can't imagine a dealer is going to try to misleadingly unload a late model vehicle with known problems.