Ignition wires jumping through insulation usually happens first when the bike is under load. When the bike is light on load, the path of least resistance might still be accross the plug-gap. As you load up the engine a little more, the insulation starts to look more appealing. Over time, that arching will wear out the insulation even more and it will happen more frequently.
Not at all uncommon. Try re-routing your wires away from metal a little more, or wrap it with appropirately sized rubber hose, cut lengthwise as an interim solution. Ultimately though you'll want to replace the leads.