If it is only on the one side, and in the same place on all the trucks with this problem, it is likely a stamping problem. That is a very large panel to stamp out with curves and contours, mainly the flare at the wheel opening. The dies used to stamp these panels are enormous, and a flat sheet of galvanized steel is placed in to be stamped. The die has what are called draw beads around the outside edge which engage the sheet to hold it while the die draws out the form of the panel. Of course there is a lot of stretching and heat going on during this draw cycle. They use oils or coolants to aid in this process, but sometimes the draw beads might show signs of galling. If this happens, it will stretch high stress areas, making the metal thin and weaker in these areas. The draw beads are there to hold the sheet until the main draw takes over, so there is no overlap or crinkling. Most of these dies have a nitro cylinders that act like air springs that aid in the draw, which could have been lower than optimum pressure. Another possibility is they didn't have the setup correct (shut height) for a certain amount of panels during a run, since others seem to be having the same problem?
My guess, by what I can see from the pic, is it may be a stamping die issue? Either that or the guy installing that side of the truck box is using a rubber hammer?