Material first ignited, source of ignition/heat, and the circumstances that bring them together. You've heard that before. Use the same principles here, and an engineering analysis will lead you in the proper direction.

First, your charger supplies the phone (or other device) at around 5 volts, and only a few watts, for a relatively low level of power. A charger is simply a small 120/5V transformer. It may take a little effort, but you can obtain, either from the manufacturer's specification or through lab measurement, the magnitude of the charger's "source impedance". It will be relatively high. Then add in the impedance of the cord and that of the connection. This will be relatively low, and you might be able to disregard it for purposes of calculation.

Using Ohms law (this is why it's important to us engineers), you can calculate the maximum possible short circuit current that can occur at the micro USB plug end. That number will be pretty low.

The worst-case scenario is probably some type of short in the end of your micro USB. Calculate the maximum wattage that the worst case short circuit can generate under the given conditions. Now you have a source of power and a magnitude to go with it (and the calculations to prove it).

Power translates to heat. You will need to determine the nature of the specific material involved, how the heat flows and how it is transferred, so that you can arrive at a max. temperature that you can achieve under these conditions.

With me so far? We're almost done. In this case, your engineering analysis has shown that the plug end will get only to a temperature that is warm, not hot. Ignition can't occur under this scenario, so we're finished.

Sure, there was failure of some type to cause the low heat. You don't even need to know what the failure was or why it occurred, so no further analysis is necessary. The manufacturer or failure analysis engineer may wish to examine it for design improvement purposes, but the O&C investigator doesn't care about a failure if it didn't cause a fire.