Sorry for your problem. The rubberish substance along the edges is a very impressive material. The characteristic we love about it is the fact you can smash a piece of it with a hammer or put it in a vice for days and it will always spring back to shape. This is important because we don't want it to wear and become lose over time. One of the other factors it has is very stretchy when cut to such thin widths.
What happened with a few ZAGGmates is that the workers were sliding their fingers down the edge as they installed it and in attempt to speed the process up, some workers would do the straight edges, then as they approached a corner, they would go on to the next edge. Finally, when done, they would push the corners in place. Seems like a nice quick way to do it, but the problem with that is you have a material that wants to spring back into shape (like a rubber band) and less adhesive in that spot because it has been stretched. Not a good combination. Most of these defects are obvious right out of the box and we've swapped them already. It sounds like you got one that took a little longer to manifest itself.
Again, sorry for the manufacturer defect. Please call customer service and they'll take care of it for you. Thanks.