What would cause this?

It's because the wires are "leaking." If you hold a test light up near the plug at night, you'll see the spark jump from inside of the boot to the test light. It makes sense if you have ever assembled ignition wires yourself. The core of the wire is folded over the insulation at the end, and then crimped under the metal connector. If the core end is too long, it will come too close to the end of the boot, and leak out the side. The heat from the misfiring is causing the boot to blister. I can pretty much guarantee you that if you pull that boot back a little, you'll find the core of the wire folded right under where that blister is. Basically it comes down to this. Motorsport wires are cheap crap, and always have been. They use inferior boot ends, and poor assembly, and just about every set I have ever installed for a customer had the same problem. Moroso and MSD wires have the same problem. If you can afford to buy another set of quality wires, that will save you a lot of hassle in the future. If you can't afford a set of wires, you can make do by buying a better quality boot for the wire. Slide the boot ends off the wire, clip the core with a pair of dykes right down to the end of the connector so that no extra is hanging out. Reassemble the wires with the new boots using dielectic grease as a lubricant. The boots that connect to the distributer generally have the same problem. Might as well go and change them out too.

Kurt