My theory as a trade electrician is what I call "strand reduction". When a cord is improperly removed by pulling on the cord instead of the cap repeatedly (like what happens when you drive off with it plugged in) you begin to break or pull wire strands out where they are connected to the blades on the plug cap. Eventually when enough strands are no longer in contact with the blade you have a reduced area for current to flow (just like if you were using a tiny extension cord), which generates the heat which will melt the plug cap and eventually start burning, catching all the plastic around it, like the grill for example.

A very loose plug connection can build heat the same way when using a high amp draw device, so make sure those extension cords are good quality and tight fitting.

It is not the exposed copper on a worn cord that starts a fire so much as a loss of contact during current flow.

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Brian
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