Two layers of insulation does not guarantee that the breakdown voltage is the sum of the individual layers breakdown voltages. For a number of reasons such as material type, dye type, manufacturing differences, voltage dependancy and contamination, the actual resistances of the layers will be different. Since this is a series circuit, the greatest voltage is developed across the highest resistance. eg., let's say one layer is 100 Gohm and the other is 200 Gohm and both are rated for 1 KV. If we apply 1 KV across the stack one layer sees 333 V and the other 667 V. Doubling the applied voltage to 2 KV nets 667 V across one and 1333 V across the other. If it's only rated for 1 KV, then it's being operated outside of its rating. I don't know what the resistance spread is for typical jackets, but I could easily envisage one batch of insulation being so much better than the average and ultimately failing because it took most of the voltage.
I don't know if there's a name for this effect, but it must have something to do with Murphy.
Many wiring standards require that all wires in a cable or raceway have to be rated for the highest voltage experienced by any one of them - unless there are special considerations such as grounded shields around the HV wires.