What causes a dc power cord to overheat?

A DC Power Cord should, in practice, never overheat. If it does, there is something very wrong. A piece of equipment is designed to draw a certain amount of current, the power supply, cord and connectors are all manufactured to withstand a little in excess of this amount and, if too much current presents, then a fuse or other safety device should break the circuit. Power cords for domestic equipment should really NEVER get hot, not even warm.