Was the electrical tape around the wire chewed up like that when you took it in or did the dealership do that? If it was chewed up when you took it in, something hit it and split it. Looks like a fairly clean cut though, which is odd. Would have had to be a pretty sharp rock or stick and I'm not seeing any other damage around it. Wires don't typically get cut, they typically get yanked out of the connectors and if that's the one that I think it is, it's tucked up high enough on the frame that you're going to have to try really hard to make it split right there.

edit: that close to the plug you'd expect a lot more damage/scrapes/scratches on nearby components if it was indeed a rock or a stick. That almost looks intentionally cut.