Okay, so if you don't know, then evidently you didn't do it. In that case, tell them there's over a 100 ways this could happen from non-digital media being used to wrong combinations of materials to poorly prepped substrates to just plain carelessness. There's no way you can know what happened when inferior products are used by your competition or heaven forbid..... HACKS. However, make sure if you repeat the order, you use something that won't do the same thing.