I've seen this twice.
Started the truck, forgot something went back inside and looked outside(to admire my truck) and the backup lights were on. Almost had a heart attack.
That would be a nice feature, if you were behind the truck and needed the lights, but I don't think it was designed that way. Of course I ran out and made sure it was in park. I can not seem to recreate it,
Any one else noticed backup lights on at startup? I never think to check it because I'm in the truck.

 

That's strange. But then again it may be related to the automatic transmission, because I have the 6spd. Maybe someone else has experienced the same problem. Almost sounds like something is out of alignment in the Gear Selector. Does your dash say that the Truck is in "Park" when it really is in Park?