Wow, that sounds a real homework question, lol.
Connector crimping... maybe.
Oxidation of the contacts in the connectors... possibly. (especially if they are dissimilar metals.)
Sharp bend causing fracture in the connecting wire*... could be.
How long and what gauge is the connecting wire???
*I once saw a UK mains outlet with only a television plugged in start to smoke. On investigation, I found that the solid-core T&E cable had been bent sharply back on itself when the socket was fitted to the backbox. (not my work!)
The stress in the wire increased the resistance at that point enough to dissipate such heat that the insulation melted. Just from a TV's current! Admittedly it was a big, old rear-screen projection TV that probably ran at a few amps.