"Hot" ground wire
Hot connections/ terminals/ wires come from two sources or a combination of them both
Resistance at some point causes overheating because the effective wire size has been reduced by the bad connection, so to say
High, abnormally high current in a circuit
Or combination of both.
So bad battery clamp connection to post, bad connection to cable, cable corroded internally, and or something causing way too much current. Bad starter, dragging engine, etc.